If you're referring to topics related to onions from 2021, or perhaps links to information about onions from that year, here are some general points that might be relevant:
In 2021, the Tor network underwent a significant transition with the retirement of "Version 2" (v2) onion addresses in favour of the more secure "Version 3" (v3) addresses. Key Research: The "20 Onion" Analysis topic links 20 onion 2021