Hey Thanos,

This was announced in early November as a part of the class reforms:
3. Shield Skill

Not being bound to any class in particular, the shield skill has been handled as an irregularity when it came to the obtainment of experience points and actions. In reviewing the identity of each class, it was decided that shield actions should be made a part of the gladiator arsenal. In order to streamline the system and accentuate class uniqueness, the shield skill will be phased out and instead take the form of actions and traits learnable by gladiators.

Please be aware that the above changes will render all gains associated with the shield skill (levels and EXP) invalid.
Actions associated with the shield skill (for example Aegis Boon) are now gladiator actions, and not connected to any "shield" class (like Sentinel was before). So there is no need to "level a shield skill" as they are a part of gladiator actions. Looking at your signature, your gladiator is level 50 so... you'll have already earned each shield action that gladiator has. I hope that clears it up!