70-74 bracket gone...

Just to clear things up a bit, yes this is actually an intended change.
There was a problem with players not having their ratings reset after leveling up.
We have applied a recent hotfix that prevents players outside of the expansion levels (70, 80, 85, 90) to queue up for Arena. On top of that, when a player levels up, their ratings should always reset.
We also plan to reset the ratings of characters that already leveled up without having their ratings reset, but that may not happen at the same time as this hotfix.

This hotfix will obviously affect players that leveled up to very specific (twink) levels to play in Arena, I completely understand how this may seem unfair to those players and will convey that feeling to the devs. My guess is this was the tradeoff they had to make to solve this issue immediately. If there's any news about this I'll let you know as soon as possible.

While it may seem somewhat unfair to players at those specific twink levels, I think that over time having these very specific brackets (70, 80, 85, 90) available for Arena is a good thing, it will allow for much better balance to take place and it will be much clearer to players at what level they should stop leveling if they want to play competitively in lower-level Arenas.