Still receiving messages after leaving a group chat?

So these are obviously iMessage group conversations as they must be if you can leave them. In iMessage group conversations, any party can also add any iMessage address to the conversation at will. Since apparently none of the other parties are adding you back in to these conversations (you would see that right in the conversation; who added you and when), it's possible that there are a lot more different iMessage group conversations (one per combination of addresses), that you are party to, than you realized on a lot of people's devices and you might just need to keep leaving them whack-a-mole-style until, by and by, you're out of all of them. If/when you do see somebody adding you back in, let them know you want to stay out before leaving again.

From Apple Support page

Send a group message on your iPhone, iPad, or iPod touch - Apple Support

Leave a group conversation

To leave a conversation, tap Details, then tap Leave this Conversation. You can only leave group iMessages that have at least three other people.

It has to be an iMessage, and has to have at least 3 others in the message to be able to leave an iMessage.