Eh, I wouldn't say it was ever really "overrated". I've said it before, but Tyranitar is more a Pokemon who has started to age rather badly rather than a Pokemon who was never that great, as you seem to be insinuating. Like yeah, in Gen 8 it was not all that good and right now in Gen 9 it's also...not great...but that's more an issue of power creep and whatnot continuing to seep in over the generations and Tyranitar has started to fall behind and show its age, not that Tyranitar itself was never great.
Back in the early generations Tyranitar was an amazing and versatile Pokemon, as it had the perfect stat build with the divine combination of excellent firepower, great bulk, wide movepool, and the hefty number of resistances it had to serve a variety of functions and allow it to fulfill any role on any team, with its severe Fighting weakness and low Speed being its Achilles heel that kept it manageable every time. It has many key resistances to Normal, Flying, Fire, Ghost, and a Psychic immunity combined with access to Pursuit in earlier gens to be an effective trapper, as well as great coverage options and offensive STAB in Rock Slide and later Stone Edge to hit hard. Sand giving a Special Defense boost in Gens 4 and beyond helped Tyranitar become an even more effective check to Ghosts and Psychics like Gengar, Alakazam, and the Latis. Tyranitar was a meta defining threat in Gen 2, 3, 4, and 5, and later a staple of Gen 6 and 7 OU thanks to these qualities. Its stat build and movepool meant it could do anything: bulky tank that can Pursuit trap, set up with Dragon Dance to be a lethal sweeper, using its sheer firepower and immense bulk to its advantage, be a powerful Choice user, and later it could be a great Stealth Rock setter and tank with utility. Its good 95 Special Attack meant it could use its special coverage options to main physical walls like Skarmory as well.
Worth noting is that Close Combat wasn't a thing in Gens 2 and 3 and so Fighting coverage wasn't that big of an issue for Tyranitar to deal with in its earliest days since good Fighting moves were borderline nonexistent in Gen 2 and the best one in Gen 3 was Brock Break.
Gen 6 and Gen 7 were a bit harsher on it but Tyranitar getting a Mega helped with its Dragon Dance sets to stay effective while its Choice and utility sets were still great. Tyranitar's Sand Stream was great not only for passive chip damage and its own Special Defense boost but also gave Tyranitar the ability to be an excellent teammate for Excadrill who could use the sandstorm to activate Sand Rush and be a deadly sweeper.
Of course it's started to fall off in Gens 8 and 9 where it lost Pursuit and its Mega and its age is starting to show with power creep being nastier than ever, but it was great for a very, very long time. Even in Gen 8 VGC and historically across many VGC metas it's been strong as a Sand setter and its STAB Rock Slide has been a force. It's not all that great nowadays in Smogon metas, but it used to be great for a long time. It's just a Pokemon who is starting to show its age. Took longer for it to start falling to power creep than Salamence or Metagross did, but yeah, it's starting to fall apart. But that doesn't mean it was never all that great to begin with, it was an amazing OU staple for many, many years for good reason.