Mobile Nvidia GPUs are nowadays basically the same as the desktop versions, the only difference being clock speeds and "skew" of chip. It's either all on your end or nothing at all. TL DR: Nothing has changed at youtube's end. This does sometimes happen, but I have not heard anything lately about current trials. Another possibility is that you have been randomly selected for testing by google and they are serving you with more compressed videos, trialing new systems for potential wider rollout.
If you use a wireless carrier for your Internet connection, many carriers compress video and send out low quality versions via various services to save on bandwidth, regardless of which version you requested. If you have a land line this is unlikely. Another possibility is your ISP may be screwing you over to save bandwidth. This happens when you go down in quality, but not usually when going up.
One likely scenario is that you have been watching 4k or blu-ray movies lately and now compressed 1080p looks noticeably worse. Sometimes they do this to save on upload time when using multiple platforms. If you are talking about a particular channel or video, the youtuber may be testing things out, or he or she may have screwed something up and not bothered to re-upload it. There have been no notable mass rollout (re-encoding) changes, and no notable changes to new videos. For years I have kept a selection of links that I check on a regular basis to check for such things. Generally, when they have big changes they re-encode previous videos of the same type. It happens infrequently enough that I haven't noticed a pattern.
The only annoying thing they've done lately is changing framerate for portions of videos to decrease file size.
They did add additional formats for a while, but it seems they have stopped doing that lately, but those didn't change existing mp4 and webm and other versions. I really wish they would add better audio and take out the hz cutoff, but that's wishful thinking. The only things they have done is add better 4k integration, but not that much. They do change a lot of things all the time, but there has been no real change in video quality for years. In short, no- there have not been any significant changes lately.