Formal release refers to the ISO release only. Upgrades are documented as being opened after the actual release, eg. on 20.04 or a LTS release, upgrades occur only after the release of the 20.04.1 ISO, and that can be days-weeks after the actual ISO release. We're still only barely weeks after 21.04 ISO, and as documented there have been issues with britney or the build infrastructure that has meant a backlog & delays on fixes hotting main repositories..
Upgrades from Ubuntu 20.10 to Ubuntu 21.04 are not enabled as it is
possible for some systems to end up in an unbootable state if they use
EFI version 1.10 - bug 1925010. Release upgrades will be enabled once
an updated version of shim is available which is compatible with EFI
The original shim bug (documented in the release notes as the reason upgrade was first disabled) has been fixed, so the upgrade was turned on... then various users upgrading were filing bugs reporting failure to boot.. another shim problem was detected & thus the upgrade tap was turned off again (to prevent more users upgrading & failing to boot...).
Decisions are made based on users experiences (& bug reports) & the moment a pattern emerges that can be easily fixed (delay upgrade until fixed) - it was enacted & upgrades disabled again (offer disappeared).. No one currently sees the option to upgrade
This is the file your machine checks to see if an upgrade is available
or if you attempt upgrade with
-d option it looks instead at this file
The Ubuntu Release Team control what appears in these files
FYI: It's again SHIM issues impacting other versions & makes... though that's just one issue being watched & part of the decision. This answer is based on discussions in #ubuntu-release in IRC