At least there was some justification for space saving in the 60s when your PDP8 gave 4k core memory. Probably not unreasonable to expect the code to be obsolete by y2k either.
Very true. I was probably guilty myself - everyone was expected to know how structures aligned, and how to pack, so "being efficient" was often just thought part of writing good code. Hopefully there weren't so many 2 digit dates being coded in the 90s though. :)
It was probably still a valid use case for comms, if not within the application. Code was still being built on x25 and oh-so-slow modem links.
There was a lot of 2-digit year coded in the early 1990s. I wrote some of it myself. The reason was compatibility with older code/data formats, and also as mentioned the notion that "this will all be gone before 2000" or that it would be updated along with everything else if it had to be.