During yesterday’s Zapdos Day, Niantic also released the first legendary Pokemon to have a legacy move; the quick move, Thunder Shock.
Special thanks to /u/MessiahX’s wife for being the first person to post a screen capture publicly.
We have immortalized the proof of this ‘holy grail’ (a shiny legendary with a legacy move) on our Pokemon Go Releases page and updated our Pokemon Go Legacy Moves page.
[CORRECTION] Thanks to keen memory of /u/apangrazio1 who remembered that Suicune (also a legendary Pokemon) had a legacy move on Dec. 8, 2017.