§ 8.6 Mandatory To Implement Stats normatively references stats that MUST be implemented for conformance with this spec.

However, the section ends by saying:

1. "An implementation MAY support generating any other statistic defined in [WEBRTC-STATS],"
2. "and MAY generate statistics that are not documented."

2 reads as the WG sanctioning undocumented APIs, which I think we should refrain from. On 1, I don't see a reason to limit documented extension to [WEBRTC-STATS]. AFAIK any spec may extend existing APIs unless the base spec explicitly forbids it.

I think the purpose here was to help clarify for readers where the conformance criteria end, which seems doable with a note. E.g. "Note: implementations might generate additional statistics beyond the ones above, as defined in other specifications such as [WEBRTC-STATS]."