In iOS 14 and later, you can individually mute a message from a contact without turning off message notifications, and this feature is available for iMessage messages as well as group messages.
To mute a message or conversation on iPhone:
Open the Messages app, swipe left on the message you want to mute in the Messages list, and tap the Ring icon.
When the action is successful, a crescent moon icon appears in front of the conversation, indicating that the contact or group message has been muted.
To unmute:
In the message list, find the message with the moon icon, swipe left and tap the ringtone icon again to do so.