Unless you encode the subtitles directly on the video stream, this is a container solution, the previous Google codecs (vp8/vp9) uses webm, which in turn is bas

Unless you encode the subtitles directly on the video stream, this is a container solution, the previous Google codecs (vp8/vp9) uses webm, which in turn is based upon mkv, so I would assume AV1 supports subtitles as easily if it uses webm as well.