One can mine Bitcoin on an IBM 1401 [1] from 63 years ago. Runs at 50 kHz and there are no binary operations so you have to simulate XOR using a subroutine operating on strings to represent bitstrings. Slow. As in seconds per hash not hashes per second. But it works!

Still. I am inclined to call that "impractical". In the same way softfloat-in-JS miners are impractical. A vast botnet of such might buy a coffee after a year or two.

At a certain point, slow enough turns into impractical, and then practically impossible.

[1] http://www.righto.com/2015/05/bitcoin-mining-on-55-year-old-...