So Just a little option on left side when searching for a games, so I do not see all games in my collections and library.

Ignore Installed Games
Ignore Games in my Collections
Ignore Games on Wishlist

This would be great to not scrool through lots of games, especially, when I have thousands of games in collections.

Here is my code, it seems to work as of now. I hope this will be usefull for you or others who see this.
So what does it do.
User needs to manually scroll page to the end to have all games in collection displayed on the current page.
The we paste this code in console. and as result .txt file downloaded with all liks leating to game page.

// Get all anchor elements with the class 'game_link'

var anchorElements = document.querySelectorAll('.game_link');

// Initialize a set to store unique href contents

var uniqueHrefContents = new Set();

// Iterate through each anchor element

anchorElements.forEach(function (anchorElement) {

    // Check if the element has the correct data-action attribute

    if (anchorElement.getAttribute('data-action') === 'game_grid') {

        // Get the href content

        var hrefContent = anchorElement.getAttribute('href');

        // Check if the href content does not contain "game_grid" and is not a duplicate

        if (hrefContent && !hrefContent.includes('game_grid') && !uniqueHrefContents.has(hrefContent)) {

            // Add the href content to the set

            uniqueHrefContents.add(hrefContent);

        }

    }

});

// Convert the set to an array and join the contents

var combinedHrefContents = Array.from(uniqueHrefContents).join('\n');

// Save the combined href contents to a file (e.g., using Blob and a link)

var blob = new Blob([combinedHrefContents], { type: 'text/plain' });

var a = document.createElement('a');

a.href = window.URL.createObjectURL(blob);

a.download = 'unique_href_contents.txt';

a.click();

So, now we suucceded at one step. We have two major left:
1. Read data and hide elements.
2. Pack the code in extension.

I am still dumbfounded about all that extension stuff and js.  It got a litte better. But I managed to fumble together a more elegant selector an extension. Currently ironing the wrinkles out. In the meantime, the script below should work for doing it manually if you use a user style extension.

/*
Copy and paste this to the console of an itch page containing game cells. Those are the 
thingies containing the image and link to a game.
Use inspect page feature from browswer and there the console.
Select one style by adjusting the comments. Or write your own.
Save the generated file and use in your user style extension.
Do not forget to encapsule the css or put a scope into your Style extension, like this example
@-moz-document url-prefix("https://itch.io/games")
{
content of the saved css(s) here. copy css*.txt cssall.txt in cmd if you want to concatenate files
}
*/
{
    let game_cell_style = '{opacity:0.07 !important}';
    //let game_cell_style = '{outline:auto purple !important}';
    //let game_cell_style = '{display:none !important}';
    let css = '';
    let list_of_links = '';
    let itch_cell_css_start = 'div.game_cell[data-game_id="';
    let itch_cell_css_trail = '"]';
    let link_re = '^https:\/\/[\\w-]+\.\itch\.io\/[\\w+-]+';
    let games_on_page = document.getElementsByClassName("game_cell");
    let temp_link_ar;
    let temp_game_id;
    for (let i = 0; i < games_on_page.length; i += 1) {
        temp_game_id = games_on_page[i].getAttribute('data-game_id');
        if (temp_game_id) {
            temp_link_ar = games_on_page[i].querySelector('.title').href.match(link_re);
            if (temp_link_ar) {
                css += itch_cell_css_start + temp_game_id + itch_cell_css_trail + game_cell_style + '\n';
                list_of_links += temp_link_ar[0] + '\n';
            }
        }
    }
    // you can copy from console too, if you like
    console.log(list_of_links);
    console.log(css);
    // you can use the line below to export a list of gamelinks instead
    // let blob = new Blob([list_of_links], { type: 'text/plain' });
    let blob = new Blob([css], { type: 'text/plain' });
    let filename = 'css.txt';
    let a = document.createElement('a');
    a.download = filename;
    a.href = URL.createObjectURL(blob);
    a.addEventListener('click', () => {
        setTimeout(() => URL.revokeObjectURL(a.href), 300 * 1000);
    });
    a.click();
}

We should as well store permalink to collection. If link is already in Database then we only updating it and not writing new Collection.

I managed to "hook" into the mechanism you use to add new items to a collection. It does not work on all subpages, but a new button appears when you add an item to a collection. It is a checked checkbox that says it will add the item to the extension's list. So if you should not want to add it, just uncheck the checkbox.

So in theory you should only populate your collection list once and then just browse and use itch regularly. 

I am currently concerned about game ids. I saw some that seemed to be recycled. If that is possible, I might have to switch to gamelinks, instead of ids. The ids at least seemed to have been recycled by the developer. But maybe I am mistaken. It was "old" ids that have recently been published. The problem with game links, developers sometimes change their name or the link of the game. It is a problem for the regular collections too, but I think this is because they create a new project with the old name.

Even if you have no time to code, maybe you could provide some feature whishes that would be cool to have. Or anyone else for that matter. I think this thread maybe got spammed by bots, the view count is way too high for only us two talking here.

I am not exactly promoting it, but for anyone willing and able to use custom browser extensions and tampermonkey scripts, there are two working client side solutions to two wanted features on my project page https://redonihunter.itch.io/itch-browse-tag-exclude

The search mods are capable of removing horror and visual-novels at the same time. This is possible, because vn are a genre. And of course there is this old feature of adding a search query to exlclude a single tag. The extension adds a box to comfortably use that exclude feature. And the tampermonkey script adds buttons to exclude any of the genres.

And as a bonus I uploaded the bare bone tampermonkey version of the exclude games feature to the project. The script features two lists. Any item on a list will by modified by a css style. Like display: none to hide (exclude) the item. It can switch to an alternative style for both lists. The alternative style is used on library pages or when toggled. You can add items or developers to the lists. And you can export and import the lists.

So the use case for the feature in this thread, just add all the owned games to list 1 and that's it.

It uses keybindings and you need to hover your mouse over the item in a browse view like library or search or browse. Default is key 1 and 2. 3 and 4 will add not the game, but it's developer. And 5 switches between the two styles.

Use list 2 for things you already know or you want to ignore or whatever. You can also export your owned games and clear the lists to do some special browsing sessions with ignore lists and later reimport your owned games. Also you do not need to ignore things. You can highlight them. It is just css styles. Stuff like scale: 0.5 does work too. Configuration is done by editing the tampermonkey script's constants near the beginning of the script.