Thanks for filing. I've merged this into the existing issue. I met with Salesforce last week during TrailheaDX about this exact topic. They're aware that it's missing and that it affects all tooling based on their completions resource which is supposed to provide all system classes. My understanding is that it should be a pretty simple fix on their end, though I don't know the timing. I'll update the linked issue as soon as I have anything to share.