Both the FSR4/RDNA 2 and FSR4/RDNA 3 mods allowed older hardware to enjoy these new features, but at a much higher performance cost. According to u/AthleteDependent926, FSR4 Redstone is much more demanding than FSR3, with the latency on their AMD Radeon RX 7800 XT at 0.13ms — almost double the reported 0.07ms achieved with the older frame gen tech, but still at an acceptable level.

I'm sure this is why they didn't bother, but they really should have just made it clear that the technique is super expensive on RDNA3 cards – they've handled this very poorly and makes us think they're just trying to push RDNA4 while abandoning RDNA3.

Reminds me of when AMD initially said they wouldn't support Zen 3 on 300 and 400 series motherboards then later backtracked after public backlash.