The register-skymap subcommand is defined in pipe_tasks (same as makeGen3Skymap.py was).
Use butler register-skymap -h to see details about the new subcommand. The options and arguments are the same as makeGen3Skymap, and a new option --config was added to allow specifying config overrides using key-value pairs on the command line.
It was mentioned in the Jira ticket that this will probably affect @yusra and @hsinfang, so please take a look.
As always, we welcome feedback and requests about the command line interface & behavior, etc.