Has internet changed since Covid?

I've had internet most of my life. There was no such thing as "allowance" and how much data you get. There was Unlimited internet and one flat fee. Ever since Covid, I've noticed a change, and I am trying to see if it is just me, or everyone else as well. I had my previous ISP for 10+ years, no issues until Covid hit. I use about the same about of internet and all of a sudden it began going ridiculously slow. Too slow to even use, so I left them for another ISP. I was assured I wouldn't go anywhere near my "limit" based on how I use my internet. That was untrue. Now I'm receiving messages on my tv and computer screen saying I went over. What happened to regular "unlimited" internet with one flat price?