With regards to archive / removal of older data: not a deal breaker, but I can see a time where we would want to purge old discussions. Depending on the nature of some communications, older discussions may have little value, and only be a potential social or legal liability.
If someone on your team is over posting in the Announcements channel, it's probably more a people problem than a software problem.
As I always say, good fences make good neighbors.
Even in testing we have seen some slack experienced users who as such are familiar with using these types of systems, accidentally posting in the wrong channel.
Thank you for the quick responses!