> The fact that Python's rule used to be 8 spaces, then got changed to 4 thereby invalidating old code really showed me how shortsighted it is to use a set number of spaces in a whitespace sensitive language.

Not sure what you're talking about here. In Python, as long as the indentation is consistent you could (e.g.) use 2-space indentation to make a code block. It's considered bad practice, and most linters will complain about it since 4-space indentation is the "golden rule."

If you're talking about the Python interpreter's interpretation of the tab character in indentation, that's different[1]. In any case, old code that has 8-space indentation using spaces rather than tabs is still good. The only issue would be old code that mixed tabs and spaces for indentation, which might not match up anymore to make correct code blocks. But, in general, it's just a bad idea to mix tabs and spaces for indentation.

[1] To the uninitiated, the Python interpreter treats a tab as X number of spaces when parsing the source code. This means that technically you could mix a block's indentation between tabs and a corresponding number of spaces. For example, if X = 4, then this code block would work:

  def function():
  <tab>something()
      something_else()