Comparing nodeinfo in GS, Mastodon and Pleroma gives this:
In GS there is an extra "api". Don't know if this is needed by design... Maybe implementing it in the same way as Mastodon and Pleroma would help to establish a standard way?
It isn't needed by design, but it was in fact a design choice :)
The standard way is to check /.well-known/nodeinfo as in https://gnusocial.net/.well-known/nodeinfo.
Also, we aren't the only deciding to put nodeinfo under /api/, Pixelfed did the same, as seen in: