The SDP used to establish a connection in WebRTC has a non-standard dialect: Plan B. Removal timeline: M93: Exception thrown in Canary. M96: Exception thrown in Beta and Stable. M102: Prior to this version, Plan B was allowed behind Deprecation Trial. With M102, sdpSemantics is ignored (you get Unified Plan no matter what). CrOS-only: Plan B was temporarily allowed up until M104.
Plan B is Chromium-only and has compatibility issues with for example Safari and Firefox. Some features are not available in Plan B. Unified Plan has been shipped-by-default since M72. WebRTC is now a W3C Recommendation.