That's how it's worked forever, basically; Windows 10 was the minimum requirement for DX12 and Vista with DX11 before that, with 6-7 year time gaps. The APIs also evolved slowly and incrementally with features between those years, but I don't think all of them were available in every operating system. DirectStorage for instance (allowing P2P DMA between your GPU and SSD) is only going to be available in Windows 11 apparently. So when/if they release DirectX13, yes, it will surely be Windows 11 only.

Speaking from personal PC gaming experience though, I don't expect this to be some major blocker, based on my experience with DirectX12 uptake in games today 5-6 years after it was introduced (it's not high outside of AAA titles, and all of them have fallbacks/alt renderers.) Not to mention the fact Windows 11 will only work on very recent hardware, so you might as well just target the "old" APIs for a good while and have a vastly larger userbase. I don't plan on upgrading for a bit and I don't think I'll miss much, game-wise, in the mean time.