no, Slow Start can be suppress and/or changed. I don’t think there is any ability that can’t be changed or suppress.Mainly when it comes to abilities there are some specific ones that only specific mons can have, that other mons can’t acquire.
There are, in fact Abilities that can't be changed or suppressed, not even with Neutralizing Gas. In
Pokémon Sword and Shield, nothing can be done to prevent Zacian's Intrepid Sword, Zamazenta's Intrepid Shield, Silvally's RKS System (though Magic Room causes Multi Attack to always be Normal-type), Wishiwashi's Schooling, Aegislash's Stance Change, Cramorant's Gulp Missile, Eiscue's Ice Face, Mimikyu's Disguise, Morpeko's Hunger Switch, or Darmanitan's Zen Mode from taking effect (though Mold Breaker and similar Abilities can break through Ice Face and Disguise). In addition, Neutralizing Gas can't suppress another Pokémon's Neutralizing Gas, though Neutralizing Gas itself can be transferred or suppressed with moves. Among the Pokémon not yet available in the game, Abilities that couldn't be changed or stopped by other means (like Gastro Acid, Skill Swap, Entrainment, or Worry Seed) include Minior's Shields Down, Arceus's Multitype, Ash-Greninja's Battle Bond, Zygarde's Power Construct, and Komala's Comatose.
Essentially, any Ability that causes a form change (like Schooling, Zen Mode, and Power Construct) or is tied to a form change (like Intrepid Sword or Battle Bond) is immovable. Comatose is an exception to this rule, but I suspect that Game Freak encountered some problems from exceptional circumstances and decided it was best that Comatose be an unchangeable, unsuppressable Ability.