Agreed… Blender dev has accelerated a lot in the past 2 years. Thankfully, the core concepts in Blender hasn’t changed a lot as I still find very old 2.x tutorials relevant and useful in most cases.
There is almost no interface change going from 2.8 to 2.9 as there was with 2.7 to 2.8, so I think pretty much all of the courses for 2.8 is still relevant. 2.9 does more to add to Blender than change it the way 2.8 did.
Not really - standard PCs will get more powerful with each generation and you will be able to do more anyway. Beginners would see little benefit… it would only be something you would consider if you were a VFX pro already. I think this upgrade to Blender means a lot more for people who operate and run render-farms. They should be able to get more out of their clusters with this addition and it gives them more options for hardware upgrade paths (getting more out of existing RTX hardware investments).