There is a BB Code text effect
fade which will smoothly fade the alpha of your text, but it isn't animated.
It's possible to animate this with scripting (by replacing the bbcode with the
start value incremented from a negative value like
-10) and this would be fairly easy to do — but it is only rendered left-to-right and can't fade the text from start-to-finish like you were wanting.
It's still possible to do that with RichTextEffects but you would need to create a custom RichTextEffect. You can see the docs page for some more information on that: https://docs.godotengine.org/en/stable/tutorials/gui/bbcode_in_richtextlabel.html
It is quite a jump in complexity from using the existing bbcode, but once you have it coded you can simply type it like any other text
This example just uses the default
fade bbcode with a script to animate the text.
var counter = -20
if text_animation == true:
counter += 1
if counter < 161:
$RichTextLabel.set_bbcode("[fade start=" + str(counter) + " length=10]" + $RichTextLabel.text + "[/fade]")
To animate the fade-out transition from start-to-finish a 2nd RichTextLabel comprised of black squares ███ and the same colour as the first label's background is animated in exactly the same way but with the start 'counter' value offset.
$RichTextLabel2.set_bbcode("[fade start=" + str(counter-180) + " length=10]" + "███████████████████████████████████████████████████████████████████████████████████████████" + "[/fade]")