As someone who has spent most of his life in Europe, but lived in the US for a couple of years in the early 80s, for another couple in the early 90s and have now been back here for a year, maybe I can offer a perspective on this.
Obviously all this is tinged with personal preference, unreliable memory (especially 90-92 as I was old enough to drink by then) and the difference in restaurants even locally, as was pointed out above. But I really haven’t seen a huge change in most places, especially at the low end, with one or two exceptions.
Hardee’s, in my perception, has actually improved greatly. I don’t see much of a difference in McDs or BK, and while the choice at Wendys has definitely declined, the quality seems to be the same (pretty poor in all three cases).
The big exception, as noted by several others, is Pizza Hut. While never being what I’d consider ‘pizza’ (of any variety, american or european), it was serious treat to my virgin palette in 1982 and still very good to my somewhat more jaded one in 1992. It’s now just cardboard, based on places in WV, PA, NJ, FL and OH in the last year. It’s so bad that I’m convinced Dominos, previously the devil incarnate of 2am eats, has actually improved since even 1999 (when I snuck over for a few months and it was still awful).
I hate to say it, but Pizza Hut in the UK, France and Belgium is actually better than over here now. And it’s really not good there, either, by any means.