Unsure whether it fits with the rest, but I'd argue it is an innovative and very compelling 'standard' that is competing with everything else mentioned in this thread.
So, the basic idea is as follows: if it is so difficult to deal with the loss of the main package manager found on the mutable/traditional variant, why don't we pursuit ways to not lose it in the first place and thus try to make it coexist (somehow) with the atomic model. Enter RakuOS's hybrid design in which everything installed through dnf is overlayed persistently over the bootc-managed base system.