It so happens that toms recently tested this exact question (how much performance do you lose pairing a modern card with an older gpu, compared to the same card with a modern cpu). Full results at [0] but the short answer is that a $900 RTX 4080 gpu with a 2017 cpu will generally do 60+fps at 1440p in most games, but as low as 55 in a few.
Not super surprising for single player games since they're usually much easier on the CPU than multiplayer. I was not getting minimum 60 FPS in Warzone for example.
0: https://www.tomshardware.com/pc-components/gpus/cpu-vs-gpu-u...