I'll update this if I find more. For now, this seems to be a partial solution. It's a Gmail add on (except I don't use Gmail) that adds reminders. The options when setting reminders include cancelling it if someone replies, and to remind everyone in the thread. If you combine those two, it seems like — if you use Gmail's web client and have $23/month you're dying to get rid of — a decent way to add the feature. Source: over 1 year ago

Use https://soundiiz.com/ to migrate your playlists to other services. I paid a few bucks for one month of subscription and transferred a boatload of playlists (even ones that Spotify generated for me, like my previous Wrapped 'lists) over to YT Music. Source: 7 months ago

For those wondering what I used to convert the playlist, its TuneMyMusic. Theres a better alternative but it has more paid features. Soundiiz. Source: 7 months ago

Although you’ll want to use a service like https://soundiiz.com to transfer your playlist or songs if you added if you want to switch music service too. Source: 7 months ago

Or soundiiz.com can create that I think, but not in order (not sure at 100%). Source: 12 months ago

The good thing is, you'll still have all of your media sorted and organized on whatever hardware was hosting Plex, so you'll just need to install new software to host it. Make sure you're using something like Web Tools or Soundiiz to keep records of any playlists or collections and you'll be fine. Source: about 1 year ago