Gnome is Linux's most used desktop environment (used by default in Fedora, Ubuntu, Pop OS!, Debian, etc.).
Gnome does not officially support the system tray. Support has to be added manually using an extension (this extension is installed in Ubuntu by default).
When you close the Screen's window, it keeps running on the tray. This means that for a lot of Linux users, Screen keeps running in the background and they can not stop it since they don't have a system tray.