It depends on your company’s retention policy for historical messages. Otherwise, you’re basically signed in as that user, so you can see whatever they see… which can be a lot depending on the company’s transparency.
It would be neat if Slack reported stats on searches so that, for example, a company could better understand what key conversations should be moved into proper documents…but this is likely against Slack’s interests.
IMO Slack and chat apps are way overused in development teams. They’re there for questions like “when are we meeting for that team event tonight?” And “Hey, can you review this code before I submit?” And NOT “Can you describe, in massive detail, all the arguments that get passed to our FooBar function?”