But that's kind of a bad approach for developing software. "Sorry, handicapped person, you don't get the new hotness. But it's ok, you can continue using this old software that we super-duper promise to continue supporting."

I've been recommending my clients avoid using it for minimum 6 months post "release" regardless of accessibility.

Gutenberg has been changing constantly. When it's "released" it should only be released as a final version to developers, meaning they're no longer changing it, before being released to the public.

I've known a few people who've developed Gutenberg compatible plugins, only to have it break 2 weeks later when an "update" arrived. "Update" meaning "it's been redone entirely different".

It's been noted elsewhere here - use the official "classic editor". Gutenberg offers ZERO benefits. It's simply a different way of accomplising the same tasks.

Microsoft Outlook for Enterprises does that; it has a “lite” mode which is for people with older browsers, older computers, or have accessibility needs. However, my reasoning is a bit flawed as just because Microsoft does it doesn’t mean it’s correct.

Also, that would mean maintain both editors going forward. Two UI-s means double the chance for regressions.

And guess which one is the priority for bug fixes? Hint: it's not the old, unexciting one.

as far as I know it is not intended to be optional once finished, although I suspect someone wanting the old one back will make a plugin replacing it.

There is already an official plugin replacing it: https://wordpress.org/plugins/classic-editor/