In a recent exchange on Threads, Instagram boss Adam Mosseri confirmed that the platform downgrades the video quality of older content in certain scenarios. "If something isn't watched for a long time, because the vast majority of views are in the beginning, we will move to a lower quality video," Mosseri explained.
YouTube videos degrade in quality over time too, as they reencode from one codec du jour to the next.
Heck, even Google drive pulled that stunt where they stopped storing photos in original resolution.
Point being, none of these companies exist primarily to archive your content - they exist to monetise it.
If you want to safeguard your content in original quality, then you need to either put it on a cloud storage that you are PAYING for, or keep it on your own hardware (and with backups)