I can't leave a group message anymore. There used to be an option "leave conversation " it is no longer available

That is not the problem. Everyone is using iMessage; yet we aren't able to click "leave this conversation." Deleting the messages is not an option. Either apple needs to fix this problem or I'm switching. I can't go to sleep due to the fact I'm getting spammed by a number I do not know that is in the group message. Blocking him isn't an option either cause he will just pop up as block user.

You can only leave a group message if everyone in the conversation is using iMessage.

This feature has not gone away. Although, I have noticed sometimes it is missing for some convos. Weird! I just activate the "do not disturb" or if they are strangers just block their number. Hope this helps. Take care

You have the option to turn on Do Not Disturb on your device to not receive notifications during your sleeping hours, or you can use the Do Not Disturb option in the actual message for the conversation. Leave this Conversation is still an option if the message is all iOS devices. I'm looking at it right now in one of our family conversations. All the family use iOS devices.