With the "Xbox Extended Feature Support" option selected, you actually can map individual keys to paddles within steam. The problem is Steam Input/BigPicture has been incredibly unreliable for me within the past few months. I'm just glad I can make changes using the first-party software now.
Edit - saw your edit. Changed the post title to make it a little more clear
Online services cost a lot of money. People don't realize how much because VCs and corpos w/ deep pockets have been subsidizing most major services for a long time. Now that the free money period is more-or-less over, these services need to start paying the bills with their users - commence enshittification