Hello,
I cross-posted this on the specific plugin page where I downloaded, per instructions. But I fear it's paint.net itself so I'm asking here as well. I was using our Admin's excellent jigsaw puzzle effect for the past year and it suddenly disappeared from the Effects/Render drop down menu. I downloaded the zip file, extracted it, and when I went to put it in the Effects folder, it was still there. I was baffled!
Then it occurred to me that instead of just donating frequently, I bought pdn from the Microsoft store, which automatically does updates. I believe one of those updates caused some plugins to stop being accessible to me. I was right! I uninstalled pdn then grabbed the free version on the web, installed it, and the jigsaw puzzle plugin was back.
Sharing this in case anyone else has such a problem. I read a lot on the how to install plugins page and it has an explanation about how to do it with the MS version, but my directory tree looked nothing like the example so I couldn't really use it. My solution seems easy and it worked - so I'll just go back to donating freely and forget the version I bought from MS.