TL;DR: Please make sure to specify sdp_semantics when constructing your PeerConnection as its default value will soon change.
While the web platform has unanimously defaulted to the spec-compliant Unified Plan for several years and Plan B has become deprecated on the web, the C++ library has lagged behind with the default value being set to SdpSemantics::kPlanB. Despite Plan B being on the path to becoming legacy code, this has caused some users to depend on Plan B without even realizing it.
We intend to change the default to kUnifiedPlan in third_party/webrtc very soon.
In order not to cause subtle bugs for those who do not specify it when the default changes, we'll soon make the app CHECK-crash if RTCConfiguration::sdp_semantics is not specified when constructing a PeerConnection.
We'll temporarily make the default value kNotSpecified and then in a few more weeks we'll change the default to kUnifiedPlan and delete the kNotSpecified value.
Our efforts are tracked here: