Everfrost mobs list from the Dec 1999/Jan 2000 spreadsheet doesn't have the Miragul lich (mob list attached as image, link to sheet can be searched on forums).
The working FA quest didn't go live until Feb 2000, it was originally a broken Sphynxes quest that never worked. Brad admitted this later on.
Patch notes for FA quest introduction (Feb 2000 / Paineel patch):
https://wiki.project1999.com/Patch_Notes#Fiery_Avenger
Patch notes with congratulations for the first FA completion (March 2000):
https://wiki.project1999.com/Patch_N...rch_21.2C_2000
So realistically I think we are looking at a February 2000 introduction of the Lich. I will check the mailing lists next.