End of 2020 was the first paper suggesting it — as it states above, but there have been many many papers further elucidating the mechanisms since then.

I’m a scientist and this is now well established (that COVID weakens your immune system through m a host of interactions with T cells, regardless of if you get long haul symptoms). It’s also well established that multiple COVID infections are associated with more impacted T cells. Population wise, since most have had COVID at least once and sometimes more than once by now, it is likely one of the things contributing to the current surge of viral illness this cold and flu season.