I'm on Mojave and still getting seed alerts for High Sierra's Safari betas.
I can unenroll from the seed program at large, but don't see selective opt-outs for specific seeds/betas.
I'd think in your example, there are certain drivers, such as security updates, pushing the need for parallel betas, especially when there are groups of users that haven't moved to current versions.
Easiest method seems to just ignore those alerts when they no longer apply to your example.
Security updates. Of course.