Heh, this happened in the UK too when they introduced new shapes of coins for already-existing currency. Just like the UK, Japan businesses have been forewarned for some time and this is mostly clickbait nonsense.
The UK doesn't have nearly as many vending machines.
I've spent 2 months in Japan and never paid by cash at a vending machine, but only by my Suica card.
Very area dependent. In more rural area cash only vending machines are more common.