It's not a conspiracy theory, which is a derogatory term. Just a theory. I've personally experienced it first hand enough times over the last 20 years with Nvidia GPUs to take it as fact, though.
Many times, updating a driver brought me worse performance across most games (I.e. 3 - 10 fps on average, more in a few games). Uninstalling the driver and rolling back to the version I was previously using, always restored performance back to what it was. This, of course, has always happened with outdated GPUs. Never happened with current gen GPUs at their time.
If you want proof / evidence in shape of an actual investigation and test with benchmarks by some independent review site such as TS or some Youtuber, I don't know of any. Only anedoctal evidence. However if you search around in forums, Reddit, etc. I'm far from being the only person who has experienced and noticed this. It's an issue that's been discussed plenty of times over the last 20 years in several forums and communities, over several generations of Nvidia GPUs. People have been noticing and discussing this since the Geforce 4 Ti/Geforce FX days.
But, anyone who has never heard about this, feel free to think I'm just full of s***.