Have I lost the plot or is there maybe an accessibility bug?

No, I see it in ios 13.3. And saw it in 13 all iterations. did you try rebooting the phone

If you press any physical button, the first time will mute the ring. A second press will decline and send the call to voicemail. You can also double click a button if you're in a hurry while the ringtone plays. Power, volume, or physical home button will all do this. If you're wearing wired earpods, holding the answer button for a few seconds will also decline the call. Not sure how you do this with the wireless airpods.

Yep the double press of a physical button has ben a long standing iOs trick from I'd say when I first started using an iPhone in 2010. But no, the decline button is not gone at all.

Any better? I know you cannot go into specifics but overall is it worth it?

I think it is. Still issues with notification not clearing but I'm beginning to think it's a feature, not a bug, which doesn't make sence, but Ok So far that's the only thing I've had to report.