Come for punchcards, stay for legibility.

80 characters per line is an odd convention in the sense that it originated from a technical limitation, but is in fact a rule of thumb perfectly familiar to any typesetting professional from long before personal computing became widespread.

Remember newspapers? Laying the text out in columns[0] is not a random quirk or result of yet another technology limitation. It is the same reason a good blog layout sets a conservative maximum width for when it is read on a landscape oriented screen.

The reason is that when each line is shorter, the entire thing becomes easier to read. Indeed, even accounting for legibility hit caused by hyphenation.

Up to a point, of course. That point may differ depending on the medium and the nature of the material: newspapers, given they deal with solid plain text and have other layout concerns, limit a line to around 50 characters; a book may go up to 80 characters. Given a program is not a relaxed fireside reading, I would place it closer to the former, but there are also factors and conventions that could bring acceptable line length up. For example, indentation and syntax highlighting, or typical identifier length (I’m looking at you, CNLabelContactRelationYoungerCousinMothersSiblingsDaughterOrFathersSistersDaughter), or editor capability to wrap lines nicely[1].

Finally, since the actual technical limitation is gone, it is actually not such a big deal to violate the line length rule on occasion.

[0] Relatedly, codebases roughly following the 80 character line length limitation unlock more interesting columnar layouts in editors and multiplexers.

[1] Isn’t the auto-wrap capability in today’s editors good enough that restricting line length is pointless at the authoring stage? Not really, and (arguably) especially not in case of any language that relies on indentation. Not that it could not be good enough, but considering code becomes increasingly write-only it seems unlikely we will see editors with perfect, context-sensitive, auto-wrap any time soon.