As others have mentioned, getting way back behind the bottom bracket with arms straightened never was a proper technique. It was a compensation for terrible geometry (frame too short/stem too long/head angle too steep) and an unwillingness to stop and lower the seat. I can’t say that droppers really changed anything because we used to lower our seats via quick release decades ago. Now it’s just much more convenient.
That said, it was still possible to ride properly on old school bikes. I know the first upgrade I always did to my old bikes was to install a sensible 50mm stem.
New geo just makes it easier to get into the proper position for steeps — seat entirely out of the way, arms bent in the attack position, legs bent, hinged at the hips, and chest open and low.
On the really insanely steep stuff you do have to get back a ways, but the key is you have to be LOW and maintain weight on the bars. We have one trail that is exceptionally steep and one issue I run into with a 29” rear wheel is that my crotch must literally be on the rear tire during some sections in order to not go over the bars. Doesn’t happen on my other bike which is a 27.5F/26R mullet setup. My next full suspension bike will definitely not have a 29” rear wheel.