You never couldn‘t buy Muse. It was always subscription based.
Since Muse has been declared as „end of life“, you can‘t subscibe it any more. The only way to get a copy is a full CC subscription. But, as Muse isn‘t developed any more, I would‘t follow this path …
Muse is End of Life. Adobe Muse EOL FAQ
There will be no more updates for it. Adobe does not sell Muse as a single application. The only way to get Muse is through an All Apps plan in Creative Cloud.
I just purchsed Adobe ALL APPS and my Muse access is up and running! (8April2020)
Until it stops. That's one of the risks we take with discontinued software. The next OS update could render it useless.
Adobe has kept Fireworks around since 2012 when it was discontinued. There's every reason to believe it will be the same with Muse.