I just learned that PROTON_ENABLE_NVAPI=1 to enable DLSS got changed to PROTON_FORCE_NVAPI=1 in Proton 9.0 (including Proton Experimental). Just in case this helps someone in the future.
On another note: Feel free to let me know if you've got a good way to keep up with necessary/useful launch options. I've simply been having a look at what people use on ProtonDB. But I've come to the conclusion that a lot of the posts there use outdated or simply false launch options.