My library is quite full of apps, games, etc., most of which are obviously no longer being developed, so I've decided to delete them. But how?

You want do "delete" them? From what? Your purchase history? That is impossible. You still have purchased those items. If you change your mind in a week and want the download button again what then?

A hide button sure would be handy.

Don't you organize your stuff in collections?  You can delete each item from those.

Or is    this is about the itch app? Can't say anything about that, as I not use it.