Part Economics, part Bitcoin

Keynesian Economics makes the seemingly incorrect assumption that money needs to be slowly devalued in order to encourage spending.

Austrians would tell you that the hardest money is the "best" money and that since Bitcoin (will) have an inflation rate of zero it is the hardest money.

My question is, propose a clone of Bitcoin, but every time you spend it, other than the transaction fee, an additional fee (say 10%) that simply got burned. (Sent to an unrecoverable wallet for example)

This would disinsentivise spending further, make money appreciate faster, and make it "harder" and more scarce as time goes by. Essentially a "harder" money.

I'm not promoting shitcoinery here, but I am wondering what Austrians are so certain 0% supply growth or shrinkage is idea.

Tldr, Why is a zero percent inflation rate like BTC ideal if we can go below zero?