Requirement to log doesn't mean the record has to be in online storage though. It could easily get rotated into cold storage every month with only a unique offline password granting you access.
Banks are (or were) required, in America, to use write-once offline media for records [1].
Do you move all old files, emails, internal knowledge articles, code repos, chat messages, etc etc to cold storage? You can’t search for anything you’re looking for in OneDrive, Outlook or Slack if the data is in cold storage so are you going to develop custom applications to support the same search filters as each original source application, but which searches your cold storage?
Once a user finds something in cold storage, you have to load it back into the SaaS app to display it. Why? A variation of the following applies to all enterprise data: A Slack message has tons of important metadata tied to it besides the text content. For example Slack user Id of the message sender, channel ID it was in, file ids of any file attachments in the message, and so on. How do you load that back into Slack from your cold storage?
You can put any data in cold storage. In the real world, that often makes the data almost useless for employees.