I'm trying to clear up some confusion that I have. Arguably because I am misremembering things from way back in my beginning of AD&D days.

I seem to remember that because they were not part of the Great Wheel Cosmology. Dark Sun, Dragonlance, Ravenloft, and Ebberron settings were not accessible from Sigil. 

Then I paused and wondered if it was that they weren't accessible via Spelljammer, but were accessible via Sigil. And now I'm floundering back and forth. 

Or is it a case of WoTC is ditching the concept of the great cosmology for the Multiverse and are adding them all in with no exclusions?

You could reach them all -- if you could find the portal.

Well, no Eberron, because it wasn't an official 'verse product.

Ravenloft was still located on Greyhawk(?), Dark Sun, Kara-Tur, Mazteca, and Dragonlance were all worlds separate from one another and accessible through it.

Remember that the multiverse of Planescape was drawn from the construction done in The Manual of the Planes by Grubb some six years earlier. WHen cook did Planescape, it was all part of a huge effort to align the fact they had created all those many worlds and wanted to align them all within 2e.

The Lady of Pain was the real key to it from the developer side -- she is the logo for the original release of it, after all.

Dark Sun was a 91 release, the others were earlier (Kara-tur, Mazteca, Ravenloft) and Planescape was a 94 release.

So yes, they were all there.