(a) Lots of people only upgrade every two, three, or even more major versions. I'm dealing with clients on 8.3, and people still pop up on Stack Overflow with 8.1 sometimes! These people don't ever see the deprecated phase.
Interesting that they'd never update their clients either. I guess if that's true
there's a mentality of if it ain't broke don't fix it going on here.
Would they read change logs before upgrading, or just cross their fingers?
(b) People routinely ignore cron job output. Even important job output. They won't see the messages, and won't act on them if they do until something actually breaks.