In previous versions (including 6.5), the following command would work:
cordova plugin add git+ssh://firstname.lastname@example.org/XXX/XXX.git#HASH
This appears to be broken in v7; it results in the following error:
Error: Failed to fetch plugin ssh://email@example.com/XXX/XXX#HASH via registry.
Probably this is either a connection problem, or plugin spec is incorrect.
Check your connection and plugin name/version/URL.
Failed to get absolute path to installed module