I know that I still had access to the Beta Forums for about 2 weeks after downloading the stable version for the S26; in the final few days the banner switched to saying "The Beta program has closed."
And I decided to check one of the other associated screens, and it implied that I was still signed up for Beta, which I interpreted to mean I would be automatically enrolled in any upcoming Beta tests. (I went ahead and selected "Withdraw", since I have the stable, so it should be safe to do so without impacting getting any of the regular updates.)
