Hey there and thanks for coming to our support site.
I'm curious, are you looking for a way to mute conversations on the stock messaging app? You mentioned that you can other 3rd apps did allow you to do this so I wanted to make sure you got an answer to your question.
Re-reading my message, I see I rambled a bit. Yes, I'm looking to add a
mute feature to the stock messaging app.
You will have to talk to LG.