For instance, an app can't start using Apple Intelligence if it's compiled with an older version of the SDK that doesn't know that such a thing exists. There are some UI exceptions, such as if the OS starts rendering high-level requests like "draw a button" in a newer style. Lots of other things take specific application support, though. MacOS 14 added desktop widgets. Unless an app adds code to configure and deploy widgets, that's not something the OS can do for it. That means that Yojimbo couldn't possibly offer widgets showing, say, the 5 most recently added documents.

If you're OK with not needing or wanting the newer features, and it doesn't rely on some old API that Apple deprecated, then sure, continue to use it! It's still a fine app. But each passing year means that all its updated competitors can do new things that it can't.