Still annoying?
If so, is the problem usually the loudness of the speakerphone, or the loudness of the person who is there? I've noticed some people talk louder when on speakerphone than when on regular phone (and some people talk louder on regular phone than when talking to someone in person).
Back before mobile phones there was a tendency for people to talk louder on the phone at first, but after being reminded a few times that just because the other person is far away doesn't mean you have to shout most people learned to talk at normal volume.
I wonder if loud talkers don't get that feedback now? With old phone handsets there was pretty much only one position for them, so the mic was about the same distance from the mouth for all speakers. Talk to loud and it would be annoying on the receiving end.
But with modern phones there are a variety of positions people hold them in, which can lead to quite different mic positions. My understanding is that they do a lot more automatic gain control and other processing to try to keep the level the same despite all those different positions. Perhaps this means that the person on the other end doesn't know you are talking loud and so unless someone on your end tells you to keep it down you might never realize you are a loud phone talker.
I told him that phone speakers "make me gassy" and then he turned it off.
Don't take functionality away because of a few bad actors. That'd be like getting rid of drones because a few people are assholes.
Put rules in place to correct the bad behavior. Kicking them off planes seems fair.