If anything, the Mono runtime is what prevents heavier (ab)use of SIMD types in the standard library, or at least it causes additional required effort to make it not regress in performance since it's nowhere near as capable as CoreCLR.

You can find the overview of relevant high-level types here: https://learn.microsoft.com/en-us/dotnet/standard/simd

For low-level types and historical context: https://devblogs.microsoft.com/dotnet/dotnet-8-hardware-intr...