As of WordPress 5.5.1, you can no longer use Instagram or Facebook embeds in WordPress. This is due to changes in the Facebook oEmbed API requiring all requests to have a valid registered Facebook app with API credentials.
To bring back Facebook and Instagram embeds, you can use the excellent oEmbed Plus plugin, written by Ayesh Karunaratne.
After you install it, go to Settings > Writing in your WordPress admin, where you will find instructions for how to set up a Facebook app and fill out the API key.
Photo by Glen Carrie on Unsplash