Core gameplay has not changed much and no real new content has been added. Think maybe a new map or two since S9? Several QoL changes over the course of the seasons have been added, though.
New season mechanics now feature a set bonus passive effect. For example this season was a Legion of Nightmares passive. It works the same as Community Buffs.

So instead of needing to wear Litany and Wailing Host, you get it passively, opening up slight gear changes. They had a Royal Ring season at one point as well.

Ultimately same deal as before, though. Spam rifts, do GRs, eventually start augmenting gear, push higher GRs, repeat.
This season you can min/max a lot by moving from your typical 6pc set build into an optimized LON variant complete with augments, etc.

I have the same question as the topic creator. I read all your replies but I have another question.

Where do I have the highest change of playing with other people? Seasonal or non-seasonal character? Which difficulty? Etc?

you will find most players in season
not sure if the current season ends in 1 month or so

most players will play at t16 or at random grifts

I don’t think the new season will end anytime soon. Probably a couple of months. Also if you need/want to start playing post in my thread it’s a free service I’ll plvl you and help you get all geared up!