Spine runs great on macOS Big Sur through Rosetta 2 (at least as tested on a dev kit). It would be a considerable effort to make a universal binary and in the end it would make almost no difference to macOS users, even on ARM Macs. Given that, we will likely continue on as we have, without a universal binary, until the time that providing that would have a significant advantage.
Rosetta does have some overhead, but Spine is quite efficient and should easily hit 60-144 FPS even through Rosetta. As long as you get your target FPS in Spine, more performance from a universal binary won't provide a noticeable benefit (except for some types of exporting).
