Not sure how long it will take to get added or if it even will, in the meantime here is a workaround.
Grab this image:
https://i.imgur.com/jTGc3fj.png
Upload it to your WP install in the media library.
Add the WhatsApp link to your Social Menu, then add the following into Appearance->Customize->Additional CSS
.footer-social-wrapper li a[href*="whatsapp.com"] svg {
display: none;
}
.footer-social-wrapper li a[href*="whatsapp.com"] {
background-image: url('/wp-content/uploads/2020/01/whatsapp.png');
background-repeat: no-repeat;
background-position: center;
}
The only things you should have to change are the whatsapp.com
values as well as the /wp-content/uploads/2020/01/whatsapp.png
directory location assuming you saved the image as a different name/location in your WP install.
You can use any WhatsApp icon that you like, there are a few different places around to use one online, it should just be 24×24 pixels in dimension, white and a .png file with no background.