Legendary Creatures (p. 11). This section has been tightened up to include the following new rules:
• The opening has a new second paragraph: “If a creature assumes the form of a legendary creature, such as through a spell, it doesn’t gain that form’s legendary actions, lair actions, or regional effects.”
• The final sentence of the “Legendary Actions” section is now two sentences: “It can forgo using them, and it can’t use them while incapacitated or otherwise unable to take actions. If surprised, it can’t use them until after its first turn in the combat.”
• The final sentence of the “Lair Actions” section is now three sentences: “On initiative count 20 (losing all initiative ties), it can use one of its lair action options. It can’t do so while incapacitated or otherwise unable to take actions. If surprised, it can’t use one until after its first turn in the combat.”
The compendium page for the Monster Manual's introduction to legendary creatures is missing the errata from 2016.
Here is the outdated section: https://www.dndbeyond.com/compendium/rules/mm/introduction#LegendaryCreatures
Here is the errata: https://media.wizards.com/2015/downloads/dnd/MM_Errata.pdf
Thank you for this report. I flag this as Bug.