Mixed attackers have been in the toilet ever since Gen 3 implemented EVs instead of allowing for max stats like the first two gens.
The game has never recovered.
Uh, no?
Like, yeah, some mixed attackers have been dogshit. But there have been pretty good examples of mixed attackers.
Infernape has been pretty good across most gens (besides gen 9).
Lucario was decent for a bit, fell off but it still can go mixed.
Iron Valiant is a recent example of a good mixed attacker, yes its got really amazing stats, but still an example.
Honchkrow uses its special attack to use stuff like Heat Wave (yes, it fell off hard this gen, but before then it was commonly around UUBL-RU ranking)
Houndoom I would consider a mixed attacker (it commonly used mixed sets) and its been used in most higher tiers.
Garchomp or some others you could use as mixed attacker examples since they did use both physical and special moves.
Blaziken commonly can use special attacks such as Overheat if it really wants to.
Toxicroak goes both physical and special, and that mon has done well despite its mediocre stats.
This is just a few. The thing is, mixed attackers are just not that common anymore. Gamefreak have been making less. Sure, there are pretty shitty mixed attackers (cacturne and to a lesser extent samurott), but its just like everything.
Also idk about the game has never recovered part btw. EVs are neither a downgrade or an upgrade, they are simply a sidegrade that changes the game. Some things benefit from it, some things don't.