What good is a messaging app that keeps you from communicating with me. The whole reason we have this app is for us to communicate, quicker and easier.
"Socially unacceptable" ... maybe you are neglecting some important communication? If your answer is no then you should discuss your availability with your team about your slack hours. Its fine to trun off the email and slack. When when you are one work time slack/email make communication better/easier. You should not blame or allow slack to employ a standard you would not be comfortable with even if slack was not invented (ex: your boss calls you at on the phone at 7pm, if thats not ok then you should also tell your boss you dont answer slack after 7pm)
The postpone workflow is to switch alerts off, can be done on schedule for when you are not at work and manually for meetings etc. And it allows you to fine tune these between channels and people etc. There are a number of statuses that show to your co-workers (on holiday, sleeping, in meeting, dnd etc). Those messages are there unread for you to come back to when you are ready.
If you've read a message, why aren't you responding? It's likely not a problem with Slack that you cannot tell one of your coworkers that you will look at something or do something later.