The default world border currently lies at 29,999,984 blocks out, which is intended to fix an issue with blocks losing their collision after 30 million blocks in versions 1.6.4 and prior (see comments in MC-53390). However, it isn't actually possible to set the world border out to exactly 30 million blocks with commands even if the player wants to (especially since this collision issue itself is fixed in newer versions, invalidating the original 30 million limit anyway).

For example, the following commands

/worldborder set 59999968 5

and

/worldborder set 60000000 5

both set the world border to the exact same position, where one would expect the latter command to set the border out to be flush with the invisible boundary wall.

Side note: it may be a good idea to simply remove the invisible boundary wall behind 30 million blocks entirely (or move it out to 2 billion), especially if MC-185925 is fixed, as just about all of the major bugs what exist at such high distances are fixed anyway. The world border would still exist at its current position and could be extended out if access to this farther terrain is desired. Rewriting the game to work out to 2^63 blocks would probably not be worthwhile though (although it would be welcome).