These were undersize (in two different sizes, IIRC) for the *real* amount that plywood these days is undersize. However, I can't remember what I was looking at when I saw them. Has anyone else seen this recently?

Yes, I remember reading about the same thing - I think I might have
seen it in a flyer from Infinity tools. After checking their website,
they have slotting cutters in 5.5mm, 5.9mm, 5/32", and 3/16". Amazon
also has a 3/16" slot cutter by Freud, but I think Infinity might have
been advertising theirs to go with a rail & stile set.
Hope this helps,
Andy