Theres been no offocial samsung releases. So take all information with a grain of salt.
Sammobile is a trusted source within the community- but not samsung offocial
"Samsung is going to launch the One UI 8 beta program in the third week of May."
"We hear that Samsung will initiate the One UI 8 beta program in the third week of May. It's likely that the beta program may not be as widely available as previous programs have been. The One UI 8 beta might only be for select devices in limited markets. A wider roll out is expected by June"
@johnz237 has pretty much sumed this up. We can expect the beta for most applicable devices in June and a few devices in a few weeks. This isn't a Samsung official statement. Just note that One UI 8 is expected to be more fine-tuning than new features compared to One UI 8.
Android 16 is still in beta and not due for final stable release untill June. Although Samsung are working on One Ui 8 I can't see them starting the beta untill after June. Maybe July or August. They're not going to release the beta based on Android 16 beta.