I put a work around for this in 2.6. There are some technical differences between the toast library and the font awesome library we are using. You’ll have to use the FA class names directly in UDToast but you’ll have access to all the icons.
Show-UDToast -Icon 'fas fa-users' -Message 'Yo!'
There is also a fall back for if you have the previous syntax defined.
Show-UDToast -Icon 'Users' -Message 'Yo!'
In 2.7, I’ll try to work out a better way to make it consistent.