I had a ticket open with Microsoft where they mentioned (casually) that Classic sites will be deprecated within a few weeks. This is a big deal for us as an Enterprise! We have custom branding and code everywhere throughout our tenant.
Not to mention there are a plethora of things in Modern that don't work or are not available. I assume they'll just hack it together and load everything in an iFrame.
Has anybody heard any official news or knows of a communication that will go out describing what to expect over Christmas when MS deprecates Classic?