128 votes51 comments · The new Outlook on the web (Office 365) » Mail · Flag idea as inappropriate… · Admin →
Hey, thanks for your feedback! This option will be added back to OWA in the next couple months. We’re working on improving the service that powers Clean Up, and then we’ll add this option back!
So....i had this issue, and found a workaround to access it. Tell me if i've wrong here too.
Login to https://portal.office.com - Top right click your icon and select "My Account". In here in the top right again click the cog, then down at the bottom under "Your app settings" select "Mail".
On my view, i now have the FULL list of settings, including Clean up mailbox.