It looks like it is referring to the line
_, ItemLink = tt:GetItem()
Which is in the function mentioned by the line you first posted.
It looks like this is one of the functions removed, or being removed based on the dev comments in GameTooltip.lua.
Lua Code:
-- Temp replacements for GetX API that's been removed
-- TODO: Evaluate for removal
function GameTooltipDataMixin:GetItem()
return TooltipUtil.GetDisplayedItem(self);
end
https://www.townlong-yak.com/framexm...ooltip.lua#983
Try using the TooltipUtil route and using your tooltip frame instead of self.