Yesterday I tried to print an e-receipt for an itinerary which finished 2 weeks ago.
Got message "System error. To get a receipt, email your ticket details to UA refunds."
Which I did, and immediately got an automated reply "Your refund

will take us 7-10 days, so do not email us again before that time."
I dont know if the e-receipt would have worked while the itinerary was still live.