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.Eliot commented
This is a no-brainer, and as Alex points out, below (Sept 24 2019 0911), there's a load of alternatives to the plus sign. :-)