I think it’s pretty new, in the last month or so. It’s currently something you have to run manually with the CLI to do an import, but they should eventually have it added into the web interface and run automatically.
The important thing is the –import flag which adds files without copying them.
Personally I run the CLI command inside the immich-server container, here’s the full command to make life easier since this took me a bit to figure out, make sure the directory is mounted into both the immich-server and immich-microservices containers. You can mount it as :ro if you want (I did) to make sure Immich can only read the files and not change them.