It looks like the main things material to m20 that remain OGL "Licensed Content" are the character creation process and classes. Meaning you could write those from scratch, paste in a large portion of 5E's mechanics, and have a workable rule set.
Also not included are spell descriptions and creatures, although I don't think of those as "required" to be included in a base rule set. Note however that the Vancian magic mechanic is part on the Commons release, so the system is there, just not the spells (which a player could always use from the SRD, you just can't include or reference them).
It will be very interesting to see how other upcoming releases stack up. But my takeaway is, like yours, our valued trove of m20 content should be safe.