Recently, I started to relearn poker a bit after a hiatus from playing online cash in 2014. I used to beat 25NL with a decent winrate. I have noticed a lot has changed regarding strategy, and people at 25NL discuss things that are way above my level from back at 2014.

I used to think about my opponent's range, equity, standard ranges in certain spots, always fold to turn and river raises unless you have the nuts, and at the end of the day I posted hands in forums and participated in discussions. Now, I hear things about range vs range, being balanced, studying solvers, etc. That used to be skills I didn't need to bother with at 25NL, only if I wanted to play 100nl or something. So, I hope someone can enlighten me, like, are honing these kind of skills that I had in 2014 enough to be able to beat a standard 25NL game? At which level are these GTO concepts and playing with solvers necessary?

Online especially zoom are very hard compare with old standard. It will not hurt to start reading solver solutions now, and you might need those knowledge at anything at or above 0.25/0.5

When you are playing in games where you are treading water against other GTObots losing to each other via rake while you wait for someone to exploit.

The level of GTO robot stuff, even at zoom, is vastly overstated by people. In addition, it's not all or nothing. A lot of GTO stuff are things most players will do but not articulate in such terms (e.g. don't checkraise very often on AK9 after calling from the big blind pre), a lot of it is just a straight up mistake to do Vs almost any opponent (cbetting too much out of position), and if it tells you you need to defend some 3rd pairs with the right blockers to a river shove Vs a spot where people have no bluffs, you can fold. Dominik Nitzsche doesn't come knocking at your door to confiscate your copy of PIO.

It's never bad to know and usually good to apply.