This pitch doesn't add any new syscalls, just Span-family taking overloads. You may be referring to the doc I posted in the other thread.
I ran a similar research task (with a few rounds of refinement) resulting in Swift System API Gaps for SwiftNIO Support. Gives us a good back-of-the-envelope idea.
edit: as to your question of whether all of these can be covered by System, my current understanding is that yes, they can, though I haven't looked into every last detail here yet (e.g. setting pthread affinity). For what's covered by the current main, and near-term slew of pitches, that's in this table. Clearly event processing is the next big area and ties directly into sockets. Everything else that SwiftNIO uses definitely looks like something that we want to support or could support.