Can I develop a package without register it and only when it is on a certain version, let’s say v0.5, register it ?
Yes. The General registry used to require specific numbers for automerge of the initial registration but that was disabled in Disable the "standard initial version number" check by DilumAluthge · Pull Request #275 · JuliaRegistries/RegistryCI.jl · GitHub.
Ahh… this is why… I did remember it, but I couldn’t find a reference to it any more… thanks for the update ! (and good that the initial version check has been removed!)