I think it's absolutely fine for people to send slacks, emails, etc after 5pm.
Being _expected_ to, however, is a different story.
Being _expected_ to, however, is a different story.
I also think it's crucial to never reply quickly to non-emergencies. Just creates the expectation you're available and encourage more of the same.
That is, unless it's a situation/relationship you want to foster to establish yourself in a team, outpace someone for promotion, etc.