Debate forums change over the years. Forums you would expect to be there, may have closed down or moved somewhere else. WHile new date platforms are made available.
I don't think debating is becoming less popular, just the fact that we may not see many debates participated in here at FP, or debate forums being advertised.
Why debate when you can log into a same-opinion safe space/think tank/cesspool! Obviously anyone with a different opinion than you is lesser and isnt worth your time. /s
There seems to be a growing lack of proper debate in the world, I've met many people with this mindset and its really sad.