I ran into a similar issue where I thought I couldn't download playlists. Just check the URL I reckon it did something funny like changing to YouTube music or something.. with the correct URL you should be fine (granted my last download was probably a couple of months ago)

Edit: I just saw your other post about YT music playlist. Ignore this comment

Is this a recent change? I remember doing this some time ago.

mind sharing an example url? seems to work for me

I just did, you definitely can, if it's rejecting it there's something wrong with the URL, i bet the browser is giving you the URL of one video with the playlist as a parameter, click on the playlist so that you can see it full, the url must say https://www.youtube.com/playlist?list=XXXXXXXXX (with the correct ID of course)