As I noted within my post #9955859@lemm.ee (alternate link), I found that thumbnail generation in Element is an enourmous privacy, and security vulnerability. Thumbnails are generated server-side, regardless of E2EE settings. What this means is that the URLs that one sends would be leaked out of your encrypted chats to the server.
Have you posted a suggestion on github? I feel like this was a proof on concept during development and maybe it was forgotten about further along the life cycle.
There are existing issues on GitHub: