We announced at Ignite that we are actively working on bringing dynamic plus aliases to Office 365.
To get around existing usage, the plan is for an opt-in setting. Our ETA is to have this available for all customers by the third quarter of 2020.
I will keep you updated in Uservoice on our progress.Luke Stevenson commented
Surely you could simply have a flag on an Account which sets whether the plus can be used as a break between the user portion of an email and an optional flag portion.
It's not as though you are making this change for hotmail, where "user+flag" may be an old username.