The elytra's speed halves when travelling over explored land

Why it should be implemented:
The elytra’s main use is to cover long distances fast, which is useful for exploration. If you want to travel a frequently used route, it would be fair if you’d have to build infrastructure, such as an ice boat track, a nether highway or a railway (the minecart would need a buff).

How it works:
If a player equipped with an elytra flies over unexplored land, their velocity is approximately 30 b/s (normal speed).

If a player equipped with an elytra flies over already explored land, their velocity is approximately 15 b/s (half the normal speed).

This could be shown optically on the elytra itself: when flying over land, it fills in its texture with a map.