If people are hidden on steam. They still get messages. It's not like stuff doesn't go through, it just basically slows pop ups when your actually doing something. Like people being bored and sending out random messages and you just want to game. Just like old school, let the machine get it.
Not hidden means let's talk or play,
hidden means im busy, it's nearly never about hiding from anyone in particular at least in myeexperience.
I'd say 90% of the time I send people messages, I know they are sleeping as I do stuff on the comp at like 4am, when most of the world is in bed. I send a message and the next day it gets responded to, so they reply when they can and the next time I log on I get my replies.