Sigarda, Host of Herons

• WOTC Official Reminder Textofficial rules
  Hexproof — This permanent can't be the target of spells or abilities your opponents control.

• WWizards of the Coast2018-12-07
  Official Ruling
  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.
• WWizards of the Coast2018-12-07
  Official Ruling
  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.
• WWizards of the Coast2018-12-07
  Official Ruling
  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.

Rulings via Scryfall