Huh.
After Ptolus 5E/Cypher (well, "after" in the sense that they ran a Kickstarter campaign; the books aren't scheduled to be released until March 2021), I had hoped that we'd see the Lands of the Diamond Throne in 5E.
The two settings have an unofficial but real overlap, given the names of the non-human races, including how gnomes, halflings and elves are given the same name as other non-human, but non-traditional, races in Arcana Unearthed, and there are references to an ancient giant society far to the west of Ptolus. In my campaign, the head canon has always been that those are the Lands of the Diamond Throne.
I guess I'll just pick up the relevant 3E PDFs if I ever want to make the connections explicit.