You definitely need to think carefully about friction for repeat users on this kind of thing, though... definitely Slack is a peculiar case because of their mind-numbing user model that leads you to having multiple accounts. but I do consulting and as a result frequently get added to other companies Slack workspaces. The deluge of "helpful" messages from Slackbot sent repeatedly in each workspace is absolutely maddening and is one of the big reasons behind my hating Slack.
You also see a similar thing with mobile apps that have a forced tutorial/onboarding intro.
Just keep in mind that every thing that you force users to do as part of onboarding is going to be annoying, and especially for users which onboard multiple times for whatever reason. Even just mobile apps repeating their first-use tutorial when you get a new phone is enough to drive a man to drink.