Have a look at DF's video, they tested it. On the 2080 it is about a 13% uplift, enough to allow for 1080p30 play with DLSS with PT on. 3080 was 10% uplift. 4090 was minimal, like 1% uplift, 5080 was also like 1% uplift. So it seems like they are not nerfing it older GPUs, in fact in those more performance constrained scenarios it is helping more where on the newer GPUs it really isn't helping, at least with the specific implementation. Now of course this is the first time someone has implemented it, and I imagine it is kinda shoehorned in and not properly optimized and all that jazz, but it is indicative that they are NOT nerfing it on older GPUs.hopefully RTX Mega Geometry won't be nerfed too much on the 40 series...
Now that said, things that use tensor cores are in general going to perform better the newer the GPU because newer models have more of them, and they are faster. But it doesn't look like they are doing things to fuck over old cards.
Same kind of deal with the DLSS transformer model. The ray reconstruction version hits pretty hard, it needs a lot of compute. The net effect is that the 30 series and older do see a non-trivial speed hit when they use it. That's not because of a nerf, they just don't have as much tensor power. The 40 series sees much less hit, since they have more power.