Hi, I’m not sure how much active development is being applied to the Twenty Nineteen theme as Twenty Twenty just came out. You could always file a Trac ticket here:
https://core.trac.www.ads-software.com/newticket
Filling out the Summary and Description with your request, make sure to set the Component field to Bundled Theme. No guarantees that the icons will be added though, because if those ones get added where is the line drawn as to how many should be added.
With all that said…..there is another way to get the icon in your social menu. First, create a Custom Link for the Social Menu that you have. In the Custom Link, set the URL to the URL of your profile on Mastodon. Save the Custom Link to the menu.
Then, go to Appearance->Customize->Additional CSS and add in the following CSS
.social-navigation .social-links-menu li a[href*="mastodon.social"] {
background-image: url('https://source.joinmastodon.org/mastodon/blog/raw/master/static/logo.png');
background-repeat: none;
background-size: contain;
background-color: black;
width: 26px;
height: 26px;
}
.social-navigation .social-links-menu li a[href*="mastodon.social"] svg {
display: none;
}
As long as everything works, you should have the Mastodon icon in your Social menu and it should link to whatever URL you input in the Custom Link URL field.