Although I don’t work with animations too often, I believe this is because :GetPlayingAnimationTracks() is now meant to be called on the Animator object instead. While it’s not documented yet, it is listed as one of its methods.
for i,v in pairs(character:GetDescendants()) do
if v:IsA("Animation") or v:IsA("AnimationController") then
if v.IsPlaying == true then
v:Stop()
end
end
end