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Both developer and public beta are available. Not a myth.

If you have installed the 10.15.5 supplemental update you will not be offered 10.15.6beta1 which does not have the security fix included in the supplemental. I had NOT installed the supplemental and was offered the beta.

In your case, best to just wait for the next beta.
 
I'm in the same position, I installed the supplemental update, however would like to install the beta. Is there an easy way to get it updated?
 
Same problem here. I decided not to roll back via Time Machine so a bit stuck now. I'd hoped I could just use recovery to reinstall the pre-supplemental version, but the app store catalina installer is already updated -- and I don't have a ful ldev account (I assume the images are avail there)

So like others a bit frustratingly stuck. You'd have thought apple would have know this would happen and deferred or re-issued the beta. Maybe they will in a few days.. or may just have to wait for beta 2.
 
There's nothing interesting in this beta (I have it) so just enjoy your security fixes, whatever they are, and wait for the next beta to be released.
 
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