I gather the intent of the new impersonation feature is to lower its impact, but there are currently effects besides last_seen_at. As near as I can tell, when impersonating, you’re being that user.
Click on a pending notification? They’ve now seen it.
Open an unread post? They’ve now read it.
Knowing this, I’d only use it for troubleshooting with the knowledge of the user. I’d have no problem with users being notified and provided with a log of activity in an impersonation session.