I meant when was it declared non-canon?
I’m pretty sure it was still canon when the first bunch of episodes of the CGI series came out. An interview of Dave Filoni can be found somewhere in which he declared he loved and respected the original show and that the CGI version was a sequel not a remake (it was before the new show was aired I guess). In fact I don’t think the first series has been officially declared non-canon by Lucas and Lucasfilm, they just stop mentioning it during the release of TCW ze movie, and then they erased it from the website and stoped producing video releases of it. Curiously the second show has never dealt with the same exact time era of the first series (Anakin is already a Jedi Knight in the second show, and it ends before Anakin and Obi-Wan were sent for a mission in the Outer-Rim territories that last months), so both can co-exist without too much continuity issues. It finally was officially declared non-canon by default when the “great reboot” was triggered in april 2014.
If someone can find something official, like an interview of GL where the conflict between the two series is mentionned, could be useful !