I just upgraded to Big Sur yesterday and could not believe that the Compose Mail icon was missing. You can find it by going to Mail, View, Customize Toolbar. Grab the New Message icon (which looks like it always has) and drag it to your Mail toolbar. If you look at the bottom of the Customize Toolbar window, you can drag the entire set of Default icons to your toolbar. Ironically, that set includes the New Message icon. Why would the Default icons not be in the toolbar when you upgrade to Big Sur? You have to dig thru all these steps to activate the Default setup??? Nonsensical.