Not sure where we're using Office Discovery Service

Sam,

     I received this message as well as did a couple of my clients.  Personally, I don't have any custom code in my tenant, and people seemed to only receive this message if Microsoft detected something in their Office 365 environment was using it.

One thing it *could* be is a third party add-in/solution you have. Some Office or Outlook Add-in could be using this Service in the background and you have no idea.  If this is the case, you shouldn't need to worry about it as the vendor of the product should be aware of it as well and updating their code to use the Graph instead. The only thing you may have to do is just make sure any third-party products you may be using are up-to-date.

Just a thought I had when I received that message in my tenant as well and knew I didn't have any custom code I wrote in my environment that was using that service.