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.
In the message list, find the message with the moon icon, swipe left and tap the ringtone icon again to do so.