until they are installed can joomla know what files are part of the discovered extension and should be removed?

Probably according to the xml file, or not?

Probably according to the xml file, or not?

sometimes yes but sometimes not. There might have been an install script for example

yes but they are not installed yet...

Many of these extensions were installed some time ago but their entries were removed from the #__extension table. I tried a PR for that but gave up, There were different files, assets, images, database tables, ..

But these messages are weird.
Maybe we can add an action button "ignore at discover" and then add a "ignore" record into the #__extension tabel.

the only reliable way to remove them is to install and then uninstall

For anyone looking at this to create a pull request it is not as simple as deleting a folder as @chmst already hinted at

Maybe we can add an action button "ignore at discover" and then add a "ignore" record into the #__extension tabel.
then we would need an ignore list in the backend too otherwise one decides to install it later and is confused why its not discovered...

making a lot of work for no benefit

yes the original feature request is actually not for "ignoring" but for a help to clean up.
If would even help if Joomla would disclose where the module, plugin, or component was found (discovered) sometimes the name is different than the folder.

that would only encourage you to delete a folder which as already stated will still leave remnants behind. The only way to remove one of these extensions is to discover install and then uninstall