勿 is a literary / formal word for 'do not' — its presence makes 勿扰 sound clipped and signlike rather than conversational. The casual equivalent is 别打扰我 / 不要打扰. You'll see 请勿打扰 / 请勿扰 on hotel doorhangers, office signs, and phone settings UI.
usage
勿扰模式 is the standard translation for 'Do Not Disturb mode' on iOS / Android in Chinese. 免打扰模式 is a synonym used by some apps.
