のカスタムフィルタとしてできればよかったのだけど、

// ==UserScript==
// @name         hide-blacklivesmatter
// @version      0.1
// @include      https://reactnative.dev/*
// @include      https://nodejs.org/*
// ==/UserScript==

const $ = query => document.querySelector(query)
const hide = query => $(query).style.display = 'none'
const removeClass = className => $('.' + className).classList.remove(className)

if (location.host === 'reactnative.dev') {
  hide('.announcement')
}

if (location.host === 'nodejs.org') {
  hide('.home-blacklivesmatterblock')
}