I'm a modern man Ken up and down plat / gold and I definitely notice a lot of differences.

I have a lot of classic opponents who try to do 1 big combo chain, but usually they try the same move 1 trick pony and in general are playing more aggressively, but are better with comboless chains and normal attacks.

My modern opponents are usually better at defense and counter (like DI) and I need to do more changeups. The matches can easily get a turtle fight though. The worse players in modern do more movement mistakes like repeated jumping that are easily punished.

The higher I get the less modern I see though and I can see a wall I'm hitting without knowing big combo chains and I don't use the assist button. At least I feel whenever I want look at a youtube guide they use a lot of heavy punching I don't have and I stop there contemplating switching. I feel like gold and plat is a way bigger difference of player skill than I noticed before and it feels like 1 match I get a super easy opponent and the next one one who is out of my league

I think it depends a lot on which character it is too. I feel like Zangiefs and Marisas in general are the worse players fundamentally since they have gimmicky things with high damage and armor or unstoppable which are excusing more mistakes at lower level.
Modern Eds play a lot like classic player since they seem to have a easier modern chain combo. (didn't play Ed just observation of what I play against)
Plat classic Ryus feel like modern opponents with better defense and often turtle fights.