Sigarda, Host of Herons — ruling 1

As a spell or ability an opponent controls resolves, if it would force you to sacrifice a permanent, you just don't. That part of the effect does nothing. If that spell or ability gives you the option to sacrifice a permanent (as Desecration Demon does), you can't take that option.

Sigarda, Host of Herons — ruling 2

If a spell or ability an opponent controls states that something happens unless you sacrifice a permanent (as Mogis, God of Slaughter does), you can't choose to sacrifice a permanent. On the other hand, if a spell or ability an opponent controls instructs you to sacrifice a permanent unless you perform an action (as Killing Wave does), you can choose whether or not to perform the action. If you don't perform the action, nothing happens, since you can't sacrifice any permanents.

Sigarda, Host of Herons — ruling 3

Sigarda's ability affects only sacrifices. It won't stop a creature from dying due to lethal damage or having 0 toughness, and it won't stop a permanent from being put into its owner's graveyard due to the "legend rule." None of these are sacrifices; they're the result of game rules.

Sigarda, Host of Herons : règles, fonctionnement et interactions (MTG)

Sigarda, Host of Herons is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Flying, hexproof Spells and abilities your opponents control can't cause you to sacrifice permanents.

How it works : Sigarda, Host of Herons

Original English text — Wizards of the Coast / Scryfall

#1December 7, 2018· Wizards of the Coast
As a spell or ability an opponent controls resolves, if it would force you to sacrifice a permanent, you just don't. That part of the effect does nothing. If that spell or ability gives you the option to sacrifice a permanent (as Desecration Demon does), you can't take that option.
No other card is named in this ruling.
#2December 7, 2018· Wizards of the Coast
If a spell or ability an opponent controls states that something happens unless you sacrifice a permanent (as Mogis, God of Slaughter does), you can't choose to sacrifice a permanent. On the other hand, if a spell or ability an opponent controls instructs you to sacrifice a permanent unless you perform an action (as Killing Wave does), you can choose whether or not to perform the action. If you don't perform the action, nothing happens, since you can't sacrifice any permanents.
No other card is named in this ruling.
#3December 7, 2018· Wizards of the Coast
Sigarda's ability affects only sacrifices. It won't stop a creature from dying due to lethal damage or having 0 toughness, and it won't stop a permanent from being put into its owner's graveyard due to the "legend rule." None of these are sacrifices; they're the result of game rules.
No other card is named in this ruling.