3.5 was edition I played the most. It was a reason why I quit RPGs for nearly a decade because I hated it so much.
Every time I see another meme about how amazing 3.5 Tarrasque is, I remmember how amogn actual 3.5 players Tarrasque was the biggest joke. It was always brought up as definite proof designers have no idea how to make good monster. It was laughably easy to beat. A wizard could casually solo it, the same abilities people now miss in 3.5 amounted to ribbons. It was a laughingstock, forums had 100+ pages discussions how to fix it and general consensus was it';s beyond saving. It was first proof in 3.5 if you cannot use magic you're only good to roll over and die.
I honestly don't know if everyone claiming 3.5 Tarrasque is such a horrifying monster are trying to rewrite history or unintentionally proving what a broken, unplayable pile of garbage 3.5 was, if it's biggest punching bag is actually dangerous in a different, better designed game.
The 3.x tarrasque became a joke, but that was a result of the extensive options combined with people's system understanding - sure a single wizard could kill it, but that still needed to be played by someone who understood the system. It was a system that gave unlimited options, so if you worked out how to combine enough of them you could break the system wide open, and the tarrasque was a great yardstick for that.
Then you come to 5e's tarrasque and it's so badly designed that it's obvious from a glance that a level 1 character with flight can just hover above it and plink it down with a bow. I've seen 3.5's brought up in comparison to that, but not as an example of difficult fights in a vacuum.