As a "bklog: Deliverable" This is decomposed into smaller issues, grouped under D: Payara 6 Upgrade
Hat tip to @poikilotherm who posted in chat a link to a video in which Steve Millidge, founder of Payara, answered a question about how long Payara 5 Community Edition (which we recommend in the guides) will receive security patches: https://youtu.be/juD0XNNq344?t=1972
I've transcribed the question and answer:
Q: "For the Community Edition, when will v5 stop getting security patches?"
A: "Payara 5 in Community was basically... when we switch the main branch over to 6, which will be on the point of 6 release, then Community will march forward on Payara 6. So Payara 5 patches, if you like, will stop at that point through Community. So then it will be Enterprise where you'll get patches and security fixes for Payara 5. Payara Community essentially is a marching version so whenever the latest release is the latest release."
I didn't watch the whole video but @poikilotherm says Payara plans to switch the main branch to 6 in Q2/2022.
(By the way, #8064 is the issue where we're tracking the next 5.x upgrade from Payara. As of this writing we are advising installations to use Payara 5.2021.5: https://guides.dataverse.org/en/5.9/installation/prerequisites.html#payara )