In reply to Stephen Vaughan.

Hi Stephen,

The first thing I noticed is that the only way to disable Gutenberg is with the Classic Editor plugin. All other methods, simple one line filter in the functions.php file or a third party plugin will not work.

It was always the case that once Gutenberg was merged into core that class names, etc, were going to change, which is the reason for this happening.

It does beg the question though is the Classic/TinyMCE Editor still extant in core

TinyMCE will remain in core for a long time, if it's ever removed - Gutenberg still makes use of it and other parts of WordPress, where text can be edited but Gutenberg isn't used, will still use TinyMCE.

Automatic core team has been a bit vague on this.

Did you mean Automattic? Although some members of Core are from Automattic, like all core changes, this is a combination of voluntary developers from numerous places. The core team has, imo, been very clear about TinyMCE remaining, so I'm not sure where you believe this vagueness is from (maybe them not giving a definitive timeline, but that's probably because no-one can).