For * + * versus :not(:first-child), I guess it's a matter of style. Maybe they are a bit different in the specificity of the rule, I don't know.

margin-block-* and margin-inline-* (versus margin-{top,right,bottom,left}) are more generic and take into account the writing-mode, direction, and text-orientation.

https://developer.mozilla.org/en-US/docs/Web/CSS/margin-inli...

Notably, for instance, if you want to support both rtl and ltr languages, margin-inline-start (instead of margin-left) will behave correctly and put the margin at the correct side.

If you ask me, it's a step towards specifying/conveying the intent and not hard-coding the look.

Specificity is the difference:

* + * is 0,0,0.

:not(:first-child) is 0,1,0.

:where(:not(:first-child)) would take it back to 0,0,0.