I read a lot of short material on my Kindle Oasis, which I add to a collection to remember reading them later. I want a way to get rid of read items in this collection.
At the risk of suggesting the obvious, why not create a collection called Already Read and move the item from your current collection to this one?
I realize that Oasis is e-ink, but on the Kindle app for android, moving things from one collection to another is relatively easy. The key thing is after you move it to this collection is (if you want this) to remove the ebook from that device. That allows the ebook to remain on the cloud while saving space on your device. ON my android app, book covers for files which are available on the Cloud but deleted on the device still show up -- so you never forget they are there.
If you are worried about making a mistake, you can move collections using the Content & Devices web page.
In current versions of the Kindle software, when you reach the end of a book or article, it's marked as "READ" in the user interface.
⋮ menu to the right of each item, there's also a "Mark As Read" menu item. You can use this to manually mark items as read.
If the item is already marked as read, the menu item becomes "Mark As Unread".