Keep_The_Local_Feed_Quieter.js

// ==UserScript==
// @name         Mastodon: Keep Local Feed Quieter
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  Stop publicly replying to your own posts on Mastodon.
// @author       Orllewin
// @match        https://merveilles.town/*
// @icon         https://www.google.com/s2/favicons?sz=64&domain=merveilles.town
// @grant        none
// ==/UserScript==

(function() {
    'use strict';
    console.log("Mastodon: Keep Local Feed Quieter")
    document.addEventListener('click', function(event) {
        if(event.target.className == "button button--block"){
            const greatGrandparent = event.target.parentNode.parentNode.parentNode

            if(greatGrandparent.className == "compose-form"){
                const privacyButton = document.getElementsByClassName("privacy-dropdown__value-icon")[0]

                if(privacyButton.innerHTML.includes("fa-globe")){
                    const displayNameElement = document.getElementsByClassName("reply-indicator__display-name")[0]
                    const replyingToAccountNameElement = displayNameElement.getElementsByClassName("display-name__account")[0]
                    const replyToName = replyingToAccountNameElement.innerHTML.substring(1)

                    var currentUser = document.head.querySelector("[property~='profile:username'][content]").content
                    currentUser = currentUser.substring(0, currentUser.indexOf("@"))

                    if(replyToName === currentUser){
                        event.stopPropagation()
                        event.preventDefault()
                        alert("Use unlisted when replying to yourself.")
                    }
                }
            }
        }

    }, true)
})();