Once Whonix 16 gets released, there should be a final package upgrade notifying users that Whonix 15 has been deprecated with a link to the wiki with upgrade information.
A one time popup can be used:
Do not show this message again. - Generic One Time Popup
Also Systemcheck - Whonix should point that out.
I need to keep re-signing (vaild-until field) the Whonix
buster APT repository to allow late users to fetch the last upgrade.
This ticket serves as a reminder for that.
Once this is done, this ticket can be modified/rolled to the next major Whonix release upgrade version.