It has always been the case that when a document is created from a template it inherits the content of the template except:

• VBA code
• ribbon xml
• building blocks

Those items stay in the template and will be available to the document whilst it remains attached to the template.

I rarely build templates that contain customized ribbon code but occasionally it has been necessary. In the interests of science I just created a document from an old template I created for a client that was using Wd2010. I then compared that to an example document I created from the same template back in 2016. To my surprise the new document contained the ribbon xml whilst the old document didn't.

As I am now using O365 I can only conclude that in Microsoft's infinite wisdom they decided to change how this works at some point. Either that or they've introduced a bug.