alt Hacker NewsAny website and app can do it. Zoom / Google Meet / YouTube / Bandcamp / Spotify already does this. I don't think it needs to be hidden behind walls. Maybe a user override can be added.
In Linux, KDE's power manager PowerDevil shows if something is blocking device or display sleep for example. I don't think it's hard to add an indicator in macOS, too.
Visibility isn't the problem. As OP mentioned, you can go into Activity Monitor to easily see what application is doing this. The user just doesn't seem to have any control over it or any way to stop a particular application from doing it.