I just read the DnD Beyond article on dragons and I found the information about lairs interesting. It looks like they have split the mecanical aspects of lairs between the stat block (more uses of legendary resistance and actions) and the region around the lair, see below:
Dragons Are Even Scarier Within Their Lairs
In the
2024 Monster Manual, adult and ancient dragons are given a substantial power boost in their lairs. They can use more Legendary Actions per turn and have access to more Legendary Resistances, meaning they’ll have more offensive and defensive power than before.
Regional Effects Have Mechanical Implications
Similarly, the region containing a dragon’s lair has been given more mechanical effects than previous iterations, making traversing the land surrounding their lair easier for DMs to run and more immersive for players to experience.
For example, drinking water found within 1 mile of a black dragon’s lair can now cause the
Poisoned condition for 1 hour. Likewise, finishing a Long Rest within the chilly fog surrounding a white dragon’s lair can now reduce a creature’s Speed by 10 feet for 1 hour.
Previously, the regional effects caused by a dragon’s lair were more ephemeral, and while they could help the DM set the scene, the players didn’t necessarily
feel the impact of entering a dragon’s domain. Now, the region surrounding a dragon’s lair has become a perilous challenge to traverse, rife with the dragon's influence.