Ahh.
https://akkoma.dev/AkkomaGang/akkoma/src/branch/develop/CHANGELOG.md#removed-5
Removed
* Scrobbling support
* `/api/v1/pleroma/scrobble`
* `/api/v1/pleroma/accounts/{id}/scrobbles`
Found via: https://github.com/kawaiiDango/pano-scrobbler/issues/440