This used to work in H 18.5, but I'm in 19.0.657 now and I cannot figure out how to get lights into the Transforms tab to edit their xforms....

I don't think it's a transform tab like a transform node, but a list of transform operations like the edit node. Moving the lights interactively adds entries to the list.

Right, but I thought this used to populate with all the lights it found, whether edited or not?

It could be because of collections?

The Transforms tab doesn't understand collections the way the Sliders and Attributes tab does. Instead, it expands a collection's contents and shows the light, regardless of the presence of transforms by the lightmixer node. So if in 18.5 you had a collection for all your lights moved in to the Sliders/Attributes tab, you'd see a listing of all the lights in the Transforms tab.

Both 18.5 and 19.0 (should) work this way.