Are you sure ASS/SSA support is restricted for the mobile app? There’s no such distinction in the release announcement…
There’s a rather dated but long standing suggestion to add support for those subtitle types to Plex Web. Though I consider it’s a bit more tricky than the suggestion suggests.
If a client doesn’t support the subtitle format natively, Plex will burn the subtitle text/images into the video stream. That job depends on the CPU and cannot be handed down to the (i)GPU.
