Still getting messages despite being removed from a group. How to stop it. SECURITY ISSUE.

It sounds like you are using SMS/MMS messaging since you referred to including a device other than Apple. With any SMS messaging, these are managed by your cell provider. Apple manages group messaging with iMessage, but that is only the case when they are all Apple devices:

• You can only remove someone if the group has four or more people, and everyone is using an Apple device, like an iPhone or iPad.
• You can only add someone to a group message if they're using an Apple device. If you want to add someone to a group message but they're using a non-Apple device, you need to create a new group message. That message will use SMS/MMS by default since someone is using a non-Apple device.

Add and remove people in group text messages on your iPhone or iPad - Apple Support

As with any Support call to Apple, they will log an issue and it is not unusual for them to say there is no timeline for a fix if there is not one that would resolve the issue currently being tested in beta. I am not entirely sure Apple could provide a fix for any SMS issue and would recommend also contacting your cell provider. It is the cell provider that is sending that message to you and they are also the one's that manage the group SMS/MMS messages.

I do see that in iOS 18 Apple removed the separate Group Messaging toggle and now just uses the MMS Messaging toggle. It is actually at Settings > Apps > Messages > MMS Messaging. That includes Group Messaging and sending photos using SMS. RCS Messaging is the improved version of SMS offered by non-Apple devices, but similar to iMessage, all participants would have to be capable of RCS messaging, otherwise it would fall back to the poor experience using SMS/MMS if MMS is enabled.

More information about the difference in iMessage, SMS, RCS is available here and makes it clear the SMS/MMS is a carrier provided service. All incoming messages do appear gray like you see whether you are part of the group or not, it would just be when you send a message the green bubble would indicate if it is SMS/RCS and as always a blue bubble would be using iMessage and only communicating with Apple devices.

What is the difference between iMessage, RCS, and SMS/MMS? - Apple Support

The same problem leaving group messages happens on Android phones as well when using SMS/MMS. I included this Support Article from the Google Support forum, which explains that there is no way to remove yourself from the SMS/MMS group chat. That only solution offered is to have the entire group abandon the chat and start a new one without you. It seems ridiculous and that is why Apple created their own iMessage platform, but it is not compatible with Android. For Android it does say you can mark the chat as SPAM to stop the Notifications, so for that problem you are having, I will investigate it further how that can be accomplished on your iPhone, but that still does not remove them from the Group Chat and the texts will continue to come in just without the Notification. For now I would suggest shutting off MMS Messaging as suggested earlier and see if that stops the Notifications.

https://support.google.com/voice/thread/194120132/how-to-remove-myself-from-group-someone-else-created?hl=en

This article does a pretty good job of explaining the differences when using iMessage groups chats and those that become SMS group chats when a non-Apple device is part of the group. Once a non-apple device is added, the message bubbles will be green to signify that you are now using the SMS protocol and also the limitations of the carrier protocol for messaging, including not being able to remove yourself. The best you can do is Mute the conversation, but as you have found, there is not way to know if another person has muted the conversation, so they may still be reading those messages. This is how the group SMS messaging has always worked and is not specific to your iPhone using SMS/MMS. That is one of the benefits of using iMessage group chats with Apple devices only where you can leave the chat.

https://www.spikenow.com/blog/tips-tricks/how-to-leave-a-group-chat-on-iphone/

You can turn off Group Messaging on your device to prevent a group chat involving a non-Apple device. Just go to Settings > Messaging > Group Messaging and turn it off. This only affects messaging with non-Apple devices.