Flexible mode (details in
VP9 RTP spec) is currently disabled in the VP9 RTP payload descriptor produced by WebRTC video sender. It will be enabled by default starting from M125.
This is a change on the sender side that could affect receivers that rely on non-flexible mode or can't properly handle flexible mode.
Flexible mode is a standard VP9 RTP feature and should be supported by compliant VP9 receivers. The last fixes related to VP9 flexible mode were made in M73.