Saying only a sorcerer can cast subtle with metamagic is fine if you have one at your table. But if you don't have a sorcerer, it does not hurt anyone to allow a player to attempt to hide the casting of a spell in a social situation.

Indeed I think there are quite a few spells that RAW require V components but actually should not do so as they are obviously intended to be cast without others noticing it while you are sneaking - such as message, an Arcane Trickster's improved mage hand or ranger spells like hunter's mark. As a DM I am certainly allowing my players to cast these spells without being noticed if they succeed on their Stealth check.