On itch.io, you can add games to collections. This is helpful not only for organization, but also because the itch app will let you browse your own collections, even offline.
Collections tab, you can see a list of your collections, which
can be filtered by a search term. Clicking on any of the collection opens
it in its own tab.
Editing collections from within the app
...is not natively possible in v18. Although, you can click the
Add to collection
button from the in-game browser (if the window is wide enough), and you can
access the https://itch.io/my-collections page.