Warning? I'm afraid I don't follow. This isn't a new change. Each skill's prerequisites are there in the tooltip. For example here is what a Tier 5 skill says when a Tier 6 skill is being trained:

Unlocks at level 40.
Requires Rage specialization.
Requires 55 trains in previous Rage tiers.
Can not unlearn: Deeper skill(s) require these points.

The reason for this is the points in the Tier 5 skill is what allowed the gladiator to start training in the Tier 6 skill. Tier 6 skills requires 90 trains in previous skills to be valid options, hence the reason a Tier 1-5 skill can not be untrained. Hope that clears it up for you. If you don't see that kind of text in your skill trees please take a screen shot and let me know because that would be a bug.

Good luck in the Pit!

-Nate-