“Please keep the length of source lines to 79 characters or less, for maximum readability in the widest range of environments.”

— GNU Coding Standards, Section 5.1

Limit all lines to a maximum of 79 characters. For flowing long blocks of text with fewer structural restrictions (docstrings or comments), the line length should be limited to 72 characters.

— PEP 8: Style Guide for Python Code

RFCs also use 72 for text, see e.g. RFC 7994