Only just found this out as downgrading my iPhone 15 Pro back to 17 as am travelling in a couple of days, but I read that if you have upgraded your apple watch to watch OS 11 beta, there is no way to manually downgrade and you have to send it in to apple if you wish to do so.
I'm sure the people on here who have upgraded are aware of this but, considering I very nearly did upgrade, I'm glad I didn't.
Sorry to be the mr tentative on here, but be very mindful of beta cycles, they are not shortcuts to getting better products, they are work, and they are frustrating and a little patients is always going to be better.