Not so much about the links being external but as seen in com_csp and httpheaders plugin where external links are added to field labels there is no way to know they are links at all

Also accessibility rules state that you should "inform" a user if a link opens in a new window

A CSS style targeting links with target="_blank" nested in labels may solve the issue. The CSS style could put some fontawesome icon (e.g. "external-link") before the link.

This should work but I am no css expert

[target="_blank"]:before {
	content:" ";
	background-image:url('data:image/svg+xml;utf8,<svg width="1792" height="1792" viewBox="0 0 1792 1792" xmlns="http://www.w3.org/2000/svg"><path d="M1408 928v320q0 119-84.5 203.5t-203.5 84.5h-832q-119 0-203.5-84.5t-84.5-203.5v-832q0-119 84.5-203.5t203.5-84.5h704q14 0 23 9t9 23v64q0 14-9 23t-23 9h-704q-66 0-113 47t-47 113v832q0 66 47 113t113 47h832q66 0 113-47t47-113v-320q0-14 9-23t23-9h64q14 0 23 9t9 23zm384-864v512q0 26-19 45t-45 19-45-19l-176-176-652 652q-10 10-23 10t-23-10l-114-114q-10-10-10-23t10-23l652-652-176-176q-19-19-19-45t19-45 45-19h512q26 0 45 19t19 45z"/></svg>');
	height:0.8em;
	width:0.8em;
	display:inline-block;
	background-size: 100%;
	margin-right:0.125em;
}

Demo
https://codepen.io/brianteeman/pen/XPyXOo

I'm not an CSS expert either but I tried this with the default "atum" template and it works:

label a[target=_blank]:before {
font-family: 'FontAwesome';
font-style: normal;
speak: none;
content: '\f08e';
margin-right: .5em;
}

The main difference is accessibility. Using an empty content and putting a background image doesn't add anything to the accessibility tree. your version puts the unpronounceable Unicode in the accessibility tree

@markusmarchewa can you provide a Pull Request (+accessibility)?

See joomla/backend-template#100 (comment)

closing than thanks.