Check whether the extension is supported and enabled.
Unfortunately, I removed the loaded/unloaded singals for plugins in the host at some point and so I don't have convenient signals for that anymore :(
I will add them back but for stable and older hosts that won't really help.

BUG: 418436

• Started p-b-i, got Share entry, disabled in settings, Share entry was gone, turned it back on, Share entry reappeared
• Patched my host to not have purpose plugin (simulating older plasma), got no share entry