The getStreamById() method was removed together with many other MediaStream-related APIs in 2014:
The stub implementation has been removed from Gecko:
It is being considered for removal in Chrome:
If you believe that this is required for web compat, please reach on that bug or on email@example.com.
Agreed, we should probably remove getStreamByID, getRemoteStreams and getLocalStreams. Maybe removeStream as well although we should probably assess it at the same time as addStream/onaddstream.
I think we should try to remove the whole legacy API.
Removing that one in particular is ok now that adapter.js has a shim for it (that might be removed in a different schedule as WebKit).