I don't see what's wrong with implementing it as an add-on to the https://en.wikipedia.org/wiki/Gecos_field as the PR in question does. It's the most logical place as the location to store user information and is even easier to opt out of—you just edit a file—than choosing whether to compile Linux with/add to DKMS a kernel module.

Edit: One can see https://github.com/systemd/systemd/blob/8878df45c1a58afdfb500fdc53ec50e057a240ce/docs/USER_RECORD_BLOB_DIRS.md?plain=1#L103 for an example of a user record file and its path. Further documentation you can read at https://github.com/systemd/systemd/blob/8878df45c1a58afdfb500fdc53ec50e057a240ce/man/systemd-userdbd.service.xml#L36 and https://github.com/systemd/systemd/blob/8878df45c1a58afdfb500fdc53ec50e057a240ce/docs/USER_RECORD.md .