What was the world border like in 1.7 in Minecraft?

Solution 1:

The 1.8 world border is only an addition to the immovable, invisible world border at exactly ±30.000.000 blocks that was already there in 1.7. The "new" world border is a few blocks before that by default.
You can leave the 1.8 border in Spectator mode and by teleporting yourself, but not the 30M border.
Beyond 30M the world generation skips its last step, that means that terrain, dirt, grass blocks and (I think) ores are there, but no tall grass, trees, lakes, villages, etc.

If you want to see it yourself, you can change your profile settings in the Minecraft launcher to start 1.7. But careful: Make a new world, otherwise it gets damaged (for example all chests are empty). Also new settings will be reset (if you don't change the game directory), for example autojump will be on if you play in 1.12+ again.

If you want to see really interesting stuff at the end of the world, google "Farlands". They are hard to describe, better watch a video. Or click this link (or this archive link). They existed in beta versions of Minecraft. Some strange effects still exist at high coordinates, but it isn't nearly as noticeable.