I recently edited a post that had a source that started with at:// . Doing a search , this style of source started showing up about 11 days ago - mid-August, 2023. I haven't looked at all of them, but the ones I've seen seem to be related to Bluesky.
They seem to be of the form: at://did:plc:[hash1]/app.bsky.feed.post/[hash2]
There are at least a couple of artists that have more than one image here with this style of source, and for those two artists, the [hash1] part is the same. The [hash2] part varies per post.
Desktop Firefox has no idea what to do with these sources - they're not even clickable. It also doesn't appear to be a common URL scheme, per Wikipedia. That article mentions "mobile deep linking", which basically means that some apps make up URL schemes so you can link directly to a certain place inside the app.
If you have the Bluesky app installed on a mobile device, you navigate to e621 in a mobile browser, and you click on an at:// link here, will that open up the app and take you directly to that post on Bluesky? If that's how it works, then there's probably some value to keeping those links as sources here on e621.
Is there any way to transform an at:// link to a https:// link that can be viewed with a normal Web browser from anywhere?