The deal is probably that the change was too drastic.
And it's not like lore changes were never bitched about, but people have issues with it more when the further away the new lore deviates from the original. This is why there was also a backlash with say, the Galio change where the new one has a very different personality of the old one.
But the thing is, at least with the Galio change there is actually the old Galio in his new lore, as the new Galio is essentially a happy mask donning over the old Galio; Galio is now bright and shiny, he wants to be so, but deep down he's also troubled and unsure. So while it was a big shift in lore and character, it also kinda adds another layer on top of the old one. Now that's kinda cool.
When they completely re-write the character like many of the old champs have gone through, for the vast majority of time an "essence" of them preserve and passes onto their new iteration. But then there's the new Varus, where his old character and story is completely wiped without a trace and for the time being, seems to be completely unrelated to the new version. Of course some people are gonna react to that quite badly.
Sure, for good measure some of this backlash is probably just some crying about ""SJW"" and some passive-aggressive anti-gay sentiments that comes out of nowhere, but I would guess and say a good portion and most likely the majority of this deal, is not a part of that but rather to do with the changing in character and lore just being a bit way too extreme.
.........
That and, another reasoning is that the old Varus lore was already a very good ground work to add in the Darkin bit, in fact it was so good of a ground work you literally can do the "lore update" by just copy pasting a random Darkin's name (and for the longest time, people assumed it was named "Pallas") into Varus's old bio and it was practically done; hence many are mad because they question "like, why would you go for the long way and actually give the name "Varus" to the Darkin and do all these shenanigan when the work is already done, and now Varus is stuck in an identity limbo because of it?"