After a long and fruitless search, several dead ends, and almost giving up in frustration, I finally accidentally figured it out. For anyone else tortured by not being able to permanently uninstall/delete an unwanted iMessage app, here is how I did it. hope it saves you some time and frustration.
• Open iMessage
• Open or create a new text message
• Click on the grey App Store icon to reveal your favorite apps
• Swipe left and click on the three round dots in a bubble at the very end to edit your favorites
• Scroll down to see if the app you want to delete is under MORE APPS
• If it's not click Edit at the top right
• Scroll down to and choose the inactive app you want to delete, toggle the green on switch, and press the plus sign on the left to add it to your favorites. It should move to the FAVORITES list instantly
• Now swipe left on the app and press Remove from Favorites
• It should finally move to MORE APPS, which is the only place you can actually delete it from
• Now you can swipe left and Delete
• Press Done and the app should delete. if it doesn't delete it may be a companion app outside of iMessage that you have to uninstall normally.
Not sure why you have to jump through so many hoops to delete a 3rd party app, but at least there's a way to do it. Hope that helps!