TAG MEMBERS' EMPLOYMENT BY COMPANY
DreamWorks Animation -- 569
Walt Disney Animation Studios -- 561
Disney TVA -- 391
DreamWorks Animation TV -- 325
Cartoon Network -- 298
Nickelodeon -- 276
Warner Bros. Animation (tv) -- 269
Fox Animation -- 254
Robin Red Breast/Titmouse -- 121
Bento Box -- 88
Sony Pictures Animation -- 83
Marvel Animation -- 72
Paramount Animation -- 65
Hasbro -- 59
Rick and Morty -- 50
Wild Canary -- 47
Warner Animation Group -- 45
Original Force -- 22
PUNY LLC -- 19
Six Point Two -- 18
Universal Cartoon Studios -- 15 ...
The only company on the list above that has been continuously under IATSE contract is Disney. There was a period of time when Warner Bros. went away, and every other corporate entity is new since Local 839's charter six-and-a-half decades ago.
Nothing is forever.
But even with Disney, labor contracts have changed. Disney Feature Animation (aka "Walt Disney Productions" in the great long-ago) was under an 839 contract from 1952 until the turn of this century, when an IA contract titled "TSL" (The Secret Lab, after the secret lab in The Emperor's New Groove) took precedence.
Today, Local 839 (otherwise known as TAG) has a contract with Disney Television Animation, and the feature division has been rechristened Walt Disney Animation Studios and operates under the TSL contract, as does the remnants of the Disney Toon Studios.
As for the rest, studios come and studios go. Warners, as noted above, ceased existing for several years then came back with a vengeance in the late eighties. There was Warner FEATURE Animation for a brief while, but that Warners division died after six years ... and now there is Warner Animation Group which concentrates on pre-production work for theatrical animated features (The Lego Movie) produced outside California.
The take-away: Local 839 and Disney have danced together for sixty-four straight years, while every other corporate entity has come to the dress ball at a later time. (Except Warner Bros. They were there at the start, then got bored and exited, then returned to drink more party punch.)
And here we are.