MB-31682 was just recently closed, and ns-server is now "mobile aware" and there is a new "mobile" service section in the Couchbase Server setup screen.
However, trying to use cluster-init and pass the mobile service results in an error:
couchbase-cli cluster-init -c --cluster-name default --cluster-username Administrator --cluster-password password --cluster-ramsize 4000 --cluster-index-ramsize 512 --cluster-fts-ramsize 256 --services data,index,query,fts,mobile
ERROR: `mobile` is not a valid service
This toy build includes the changes from MB-31682: