LinkedIn isn't old enough to have anything ancient. It was launched in 2003, and even then you'd get laughed at for suggesting storing passwords in plaintext.
Doesn't mean that the infra is still ancient. What I see a lot is tech debt from migrations. Lots of times both the old and new systems have to work together for a period of time, so you leave certain legacy protocols and flags in place for the transition period and then the new system is never fully "updated" to the new standards. Pre win2k AD, file path lengths, encryption protocols, etc etc. Sure, the new system is "up to date" but the old compatibility settings remain.
