How do I find out if an instance is on an older version of Lemmy (like 0.17)?
If you open the instance in your browser, the version is displayed in the footer on the very bottom. Alternatively you can also go to https://lemmy.world/instances, and they show the version number of every instance in the second column.
A simple network sniffer is enough to figure out what sites the app is connecting to, then you can just block them on the router.
If you are using Android, you can do that easily right on your phone with PCAPdroid, it’s on the play store. No root required.