Simply put, this would lock a playlist from being edited further until a password is entered. I can’t tell you how many times my 2-3 year old has deleted all the items out of her playlists. In the iOS app it’s all too easy because “clear” and “delete” are right at the top of the playlist screen.
Same problem here:
I posted the same request before I realized you posted this:
"I have a request for Plex for Android.
It would be very helpful for an option to be added so that Playlists are locked.
It could simply be a “locked” checkbox that could be toggled in the contextual menu for that playlist
It’s way too easy to remove a movie with a swipe action from the playlists.
I delete movies off a playlist all the time accidentally. It’s a pain to have to find the movie and add it back all the time.
If not a lock, at the very least a confirmation before removing the movie.
I really need this as well, especially for Android. It’s way too easy to accidentally delete tracks while browsing (swiping) a playlist.
Need this feature!
It’s 2020 and this is still a want…
It’s 2021 and this is still a want… 
Just an idea I got yesterday when I made a playlist based on the setlist of a concert I went to. I never want this playlist to change, so I was thinking it could be useful to have an option to lock it to prevent unwanted editing? (such as adding or moving tracks)
Thanks.
My suggestion is to create a lock ‘on or off’ while editing static playlists and collections to prevent accidental changes. Especially on plexamp.
The other day I opened a playlist and accidentally swiped a track off it on my phone, and later had to find the track again and re-add it in the correct position.
I know we can setup fancy multi user permissions, but I like the simplicity of having full access through interfaces to edit if I need to.