This is an android thing to begin with, before you start complaining how htc have changed things.

Edit: it’s visible if you just have updated your One, but haven’t done an factory restore.  Or least it was at mine, but after a factory restore it was not visible.

This way work for all htc phones having 4.2 or above, from One X to One with 4.4.2

Developer options
Under Settings are now hidden, you enable it by going to Settings > About > Software information > More.. where you touch Build number 7 times. Which will give you a count down, and then say “you’re now a developer”. And Developer Options are back!

Ok, that’s all good. But, how do I disable it again? To hide it again.
Of course I’ve covered that as well. Read more HERE