How can you remove yourself from a group iMessage that you did not create?

In order to remove yourself, they have to be at least three other people in the group.

How to leave a group text on your iPhone, iPad, or iPod touch - Apple Support

If Android phones are involved, you can’t leave the group. It only works for iMessages.

If it is truly an iMessage (not SMS) then tap the people icons at the top of the thread, then tap on the "i" info icon, then scroll to the bottom and select "Leave this Conversation".

In that case, you cannot leave the conversation. What I do is to simply delete the conversation.