It's a nice piece of work! If you're interested, .NET's compiler has improved significantly since 3.1, in particular, around structs and pre-existing intrinsics (which are no longer needed to be used directly in most situations - pretty much all code prefers to use plain methods on VectorXXX<T> whenever possible). Also note the use of AggressiveOptimization attribute which disables tiered compilation and forces the static initialization checks your readme refers to - removing AO allows the compiler to bake statics directly into codegen through tiered compilation as upon reaching Tier 1 the value of such readonly statics will be known. For trivially constructed values, it is better to not store such in fields but rather construct them in place via e.g. expression-bodied properties like 'Vector128<byte> MASK => Vector128.Create((byte)0x80)`. I don't remember exactly whether this was introduced in Core 3.1 or 5, but today the use of `AggressiveOptimization` flag is discouraged unless you do need to bypass DynamicPGO.

You also noted the lack of ability to express numeric properties of T within generic context. This was indeed true, and this limitation was eventually addressed by generic math feature. There are INumber<T>, IBinaryInteger<T> and others to constrain the T on, which bring the comparison operators you were looking for.

In general, the knowledge around vectorized code has substantially improved within the community, and it is used quite more liberally nowadays by those who are aware of it.