He is not any one particular specialization as those are just more gameplay aspects than lore.
However, within the gameplay itself it does seem like Blizzard loves to tie Uther’s name and character aspects to the Holy specialization, specifically the mastery being named Lightbringer which lends our healing playstyle to being a lot like how Uther played in WC3 and HotS, getting up close in melee and mixing healing troops with damaging hammer strikes and holy fire.
Uther’s personality also perhaps resonates with Holy the most, as he was always described as a compassionate person who did not relish in battle. Only seeing it as a necessity to restore order and peace.