I am not Spock wrote:
A lot of the episodes were written by TOS veterans. I don't know when Roddenberry decided to turn his back on it.
The "What is canon?" memo of 1989 coincided with several pertinent events. Filmation was being sold off. The output of their TV series backlog was in a state of ownership flux; it was not definite that TAS rights would revert to Paramount/Viacom since it was originally a joint production of Roddenberry's Norway Corp., NBC Children's TV and Filmation. TAS had not been on air for some time so it was not accessible to TNG's writers or viewers. David Gerrold and DC Fontana were suing Roddenberry over co-creatorship of TNG; it was politically sensible for the lawyers to dis-count their contribution to other material. Larry Niven's "Known Space" universe (and kzinti) was about to be licensed as a RPG.
When Richard Arnold defined "canon" from 1989 on, he would specify "live action". Until then, publications such as Lincoln Enterprises' TAS newsletters showed that GR was formerly a big supporter of TAS.