Also: the optimal line width for prose seems to be 50-75 characters (including whitespace). Source: https://baymard.com/blog/line-length-readability

Assuming you're using a language where indentation is either mandated (e.g. Python) or recommended (e.g. C with "One True Braces"), with 4-spaces indentation (here again, no experimental evidence, but this seems to be the norm nowadays), and you don't abuse cyclomatic complexity in your functions / methods, you have let's say between 2 and 4 indents on the left so 80 is close to the optimum.

Note: up to 100 is probably still OK from a readability POW. But this assumes that all readers are able to increase their window size from the default 80. That's probably true for people using GUIs, but may still be problematic for people with sight issues (e.g. people over 50 or with more serious medical conditions...).