Sacred Cow
There is True Art, there is Fandom Heresy, there's even Complaining About People Not Liking the Show and then there is this trope.
A form of Unacceptable Targets, a Sacred Cow is something that is met with near universal approval and are outside the bounds of criticism even if flaws are present.
The Trope Namer is based on the popular understanding of the elevated place of cows in Hinduism, being a symbol of wealth, strength, abundance, selfless giving and life; and no harm should come to them. In some cultures, especially in India, the slaughter of cattle may be prohibited and their meat may be taboo to consume.
When it comes to fandoms, a sacred cow is a work or something within a work that "everyone" is supposed to like and is "granted immunity to any form of criticism in any way", so to speak. Covering anything from a whole series or fictional race right down to props used by said fictional race within that series' universe, the Sacred Cow is considered so either out of respect, nostalgia or plain rabid fanlove. This is usually the preserve of a rampaging fanbase but it also can apply to almost everyone including casual fans and even non-fans.
Unfortunately, this also means a fair critique cannot be made against it at any point in time as it'll be met with the same revulsion as Squick-inducing Shipping or trolling. Being deemed as high octane Flame Bait and causing an extreme form of Internet Backdraft almost anywhere, if one is shown in a bad light no matter how fair, expect retribution from anyone nearby to be overly harsh with the perpetrator Gannon Banned soon after. Of course, this can overlap greatly with your average internet troll so sometimes the accuser is painted as such and dealt with accordingly. An unfortunate amount of Fan Dumb, Fandom Heresy, Hypocritical Fandom, Nostalgia Filter, and The Law of Fan Jackassery tend to come from this.
A Sub-Trope of Unacceptable Targets. Compare with True Art; where most of this arguably comes from, and Complaining About People Not Liking the Show; in which "everyone", or at least a good amount of people will be doing. Similar to but not the same as Fandom Heresy as sometimes non-fans will agree with their sentiment. Contrast with Public Medium Ignorance, in which unlike this trope, you're supposed to dislike the work; even if you didn't watch it.