I would say, that now, they are a bit irrelevant for sure. I will tell you what happened in my case.

I was solving the a2Oj Div2E ladder. I did like, 10-12 problems maybe? from that div2E ladder. Well, that might sound as a pretty small number, but yes I have some insights for you, which may/may not be useful.

The problems from that ladder are really good. And, are hard as well. And those problems did teach me new things. But now, Codeforces has changed its taste.

These days, there are very few problems, which involve using some good data structure, and solving the problem by that. Like, for example, in that div2E ladder, I got to see one problem that was based on merge sort tree. And there are plenty of segment tree problems in that ladder as well.

But, problems that are asked by Codeforces these days are more "thinking oriented", and "ad-hoc".

You don't need that many data structures, but yes, good observation skills are a must for now. Its not like that those a2Oj problems are bad. Codeforces' problems were great, and are great.

But, just, the taste of problems have changed, and thus, practicing the old problems might not yield you results in the recent rounds. Yes, I do specifically talk about div2E ladder, but I think that might be true for div2C also maybe?

I hope I was able to convey what I wanted to. And maybe this could be a little useful for you. Good luck!

There are advantages to training on newer problems because problem style changes.

But in my opinion it becomes important only at a higher level. When you are still learning to solve basic problems, the difference isn't so pronounced.