Plex Media Server can already transcode ASS to SRT. In your client, check your subtitle settings and make sure it is set to “image only”. Since ASS can contain fancy formatting info, leaving that setting to “auto” will result in PMS burning in the subtitles to keep that formatting.
Also, Plex’s implementation of exoplayer is not a direct copy of what Google releases, so not all features mentioned in Google’s release notes apply to Plex.
