How can I make the older protocols available in mockup mode? I have been reading the docs and this blog and this particular point is not explained. I have also tried to build older versions of Octez (both manually and with Docker) but I run into different issues and I wonder if there is an easier solution.
Sorry but many Octez client commands are not maintained after a protocol is replaced on mainnet. The commands that are kept are the ones for fetching information from old blocks, these can still be useful on mainnet; all commands for modifying the state of the chain are removed. You can read more about this here. For this reason, the mockup mode of Octez client is not maintained for old protocols because you would not be able to do anything after the activation of the old protocol.
Reverting the freeze commits in the git history of Octez will probably not work and it is probably easier to use old Octez releases.