I've gotten the impression that some places force you to be online on Slack all day and therefore receive notifications. Maybe the Slack admin can see if you disabled notifications even, which could mean you get reprimanded. But I'm not sure.
We are sadly utilizing this heavily. Disabling notifications in those channels is not permitted as essentially you are required to reply as soon as possible. They exist for the sole reason of instant communication, which I find counter productive but product-xxx people LOVE.
Those guests will not play by your rules. Those guests will play by their bosses rules, who might err on the side of yelling at you at first chance. Things like "QA didn't catch bug X" people pinging on a friday afternoon, rather than going in the process of opening a ticket.
Again you cannot ignore them, and you cannot change this, because these people are paying you. Usually it's team leads that are present in those rooms, so they get all the shit thrown at them at random times, and can only tell themselves "hey I 'm paid better than average so it's part of the job right?". No, no it's not part of the job. Whoever made this decision is dumb, and Slack unfortunately catered to the wrong feature requests and made this too easy to happen too.
Since I am seeing how trigger happy our product managers are to invite the client org's PMs to our Slack, I can only dream that in an ironic twist of fate when Slack's product people reviewed this feature request they immediately loved it too and on a friday evening pinged the slack team leads to hop on it :p