A few - well, more than a few, when you think about it - Gloss-Finished models continued to be produced through the late 1960s and well into the 1970s.
A few examples? The Palomino Five-Gaiter continued in Gloss through 1971, the Running Mare and Foal was produced in Gloss Dark Dapple Gray through 1973, and the Horned Hereford Bull was released in Gloss throughout almost the entirety of the 1970s. The Dapple Gray Old Timer and the Brahma Bull didn’t transition to a Matte Finish until much later in their runs.
What didn’t happen as often after 1966 were entirely new releases in Gloss Finishes.
The first, obviously, was the Charcoal Running Stallion in 1968. All of his other original release colors - Alabaster/White, Red Roan, and Black Appaloosa - were Matte-Finished, and at least nominally realistic. I don’t know why they went with Gloss Charcoal for him, other than he looked awesome in it. (A perfectly valid reason, by the way.)
Some of the Family Foals were apparently rereleased in Gloss in the late 1960s, though the reasons why are unknown. Perhaps by customer order or request, as I’ve speculated with other Post Production SRs/Reissues?
In the mid-1970s, some of the newly-released Stablemates came in Gloss Finish variations. The rumor heard way back when was that in the mid-1970s, Breyer was trying to use up the last buckets of the old Gloss varnish lying around, and as a consequence some of that may have been applied to random batches of early (1975/76) Stablemates.
The same theory also apparently explained that coveted handful of Gloss Chalky Dapple Gray Shires, who appeared around the same time, per a ca. 1975 sales flier:
What’s always baffled me are the Gloss Misties. What was the reason for releasing her that way initially in 1972, especially after obviously committing to a more realistic - and more Matte-Finished - aesthetic five years earlier? Other than she, too, looked awesome in it?
My hand-airbrushed, possible Salesman's Sample or just super-early Gloss Misty. Beautiful little girl, isn't she? I never tire of looking at her.