In D&D5E, all wizards select a subclass - most of them are school "specializations", but others exist. Taking a particular school does not bar the character from casting the full range of wizard spells1, but give a few special abilities related to the type of magic they're best at.
True enough, your character's favored spells would be less effective against creatures with truesight. However, by the time a wizard reaches that level, there should be plenty of other spells in his arsenal. As long as you don't neglect other schools, your character should be fine.
1In previous editions, specialist wizards lost out on entire schools of magic to make them better in their selected school.