That's just a specific element of your workplace's culture then. At jobs I've worked at it would have been much more reasonable to say "I didn't see that email"

That's just a specific element of your workplace's culture then. At jobs I've worked at it would have been much more reasonable to say "I didn't see that email" than "I didn't see that Slack message." Like all communication methods, the standards that are set around them are more important than the mechanism itself.