Makes me so happy with having picked Spine up so recently to see this coming soon. Looks like a better baked and polished version of the physics motor/dynamic physics features that were in Creature2D (seems to have been discontinued ) which I used in the past for a bunch of game animation stuff, but I can't wait to get to play with this stuff with Spine's way better runtimes compared to anything else I used prior.
Was just watching Armanimation's stream and this looks more amazing than I could have imagined. V4.2 seems like it could make so much marketing bait material to advertise Spine to 2D game artists and 2D animators, and I mean this in the best way possible. Hope you get a bunch more new people pick it up for this awesome update.
A question, will third party runtime developers be able to support the beta features while this version is in beta? I know you said that the features would come to the (official I presume) runtimes in order of popularity (aside from libGDX runtime dropping first for obvious reasons) but I don't know how well this translates to runtimes that are not maintained by Spine team?