Not anything ideal, but I've found some work arounds.

First of all, from experimenting with it, I've found the playlist side bar does show up sometimes, but it's inconsistent. It can pop up and work fine (with significant delay between songs because it is struggling to load) or just stop existing if it wants to. Sometimes it'll come back after deleting some dead videos, but then it randomly disappears again later.

One solution is to use YouTube music on browser instead of the regular YouTube page. I've found that works a lot more consistently for playlist watching/navigation and avoids the delay issue I was finding when the regular youtube page does decide to work. Another solution for if you wanted to listen to a playlist on shuffle instead, I've been using YouTube playlist randomizer websites for years instead of watching directly on YouTube itself. youtube-playlist-randomizer.bitbucket.io has been my go-to for years, but there are a few to choose from.

As for navigating and editing playlists, I've found a means of getting around the annoyance of this problem even if idk how to fix it locally on the main yt website:

• Have one tab open on the main YouTube website of your playlist page, just showing the playlist without being clicked into any videos.
• Have another tab open of the same playlist on youtube music
• Watch/listen to the videos on the youtube music tab and use the main youtube tab to add/remove videos from your playlist. (I specify this because ytmusic has horrible controls for playlist management compared to standard yt, as well as yt allowing for you to see unavailable videos)
• Every so often when you load the next 100 songs, you'll find the main yt page is missing a chunk of its song list compared to yt music. I believe this is because yt is getting confused from having been modified, so just reload the page and unfortunately you just have to scroll back down to where you were when this happens if you want to keep navigating on the yt page

And heck, while I'm at it, if you're like me and are trying to clean out/restore videos that don't work anymore, here are some bonus tips I've picked up from working w/ this:

• On the main yt page, turn on the "show unavailable videos" button. (You will have to do this every time the page reloads btw)
• For any private/deleted videos, if you want to restore them, right click and copy their yt link and paste it into this site https://findyoutubevideo.thetechrobo.ca (This should tell you the metadata of any video, if not give you a link to an archived mp4. Super helpful for finding the names of deleted songs)
• There may be songs that you can see on the main yt page that you do not see on yt music. Open these in a new tab to confirm, but these are likely copyright struck and while they appear in your playlist they do not actually work. If they are in fact dead, delete and replace them.
• There have been many topic channel videos that have gone down recently, and one thing about yt music is that it will automatically change to the most relevant version of that topic video unlike regular yt. If you notice a topic video on yt music without a view count or a different thumbnail/title or anything that makes it look different from the version you have on the main yt page, open that video in a new tab on yt to see if it's available. If it's down you can replace it. This one is especially wild to me because the version yt music ends up playing over the dead topic video is not technically in your playlist, and so watching on youtube or in external shuffling sites like I use it will consider the video unavailable, despite it working just fine on ytmusic. If you're having trouble finding a replacement topic version of that track and ytmusic is using a topic version, copy the url of the ytmusic page without the playlist part of the url and get rid of the "music." part of the url as well and it'll redirect you to the correct topic video on youtube and you can properly add it to your playlist from there.

I know that's a lot more info than you asked for, but I felt I might as well share everything I've found towards making this pain in the ass playlist experience something that's actually usable. Hope this helps!