If you don’t care about extensions, try some of those “DE default” browsers that never seem to be actual defaults. The ones like GNOME Web, Falkon, Konqueror, Midori (it was the default on one XFCE system once).

Does this have anything to do with Matrix encrypted chat protocol?

Yeah, looks like it scans everything in your browser. How Orwellian.

(Not sure, but probably)

The phenomenon is called Gell-Mann amnesia