Does Just Mercy Have a Post-Credit Scene?

Yes (maybe). There are photos and descriptions about what transpired since the end of the story. However, there is inconsistency in when these extras are shown. In some theaters they come before the credits. In other theaters they come during the credits.

Either way, just stick around until you see the photos and descriptions end.

The credits run for approximately 4 minutes.

Before the credits run there are 5 minutes of photos and descriptions about what transpired since the end of the story.

The credits run for approximately 4 minutes.