You used to be able to opt-in / opt-out through a Forum post but this system is closed and according to “newest” info (info from 2023) will come back some day with a different way to opt-in.
Yeah, we do not run a platform-wide beta program anymore for new features, we run per-feature betas now. They are usually announced in the Updates category.
For the 700 player feature specifically, the latest info I got about it is that we are not touching that allow-list anymore because that feature is not stable enough to expand at this time, yet we didn’t want to take it away from users that already had it.