Documentation

My boss just asked me why we had coded in a specific way (2 year old code). I had to search in different slack channels, old commits and old jira stories to find any documentation on this. But i was unable to find anything. Though i am not sure I didn't miss anything. So now we don't dare to change the peice of code since we might have had a reason for doing so 2 years ago when we coded it. This absolutely sucks... I guess all tech companies have the same problem with poorly documented code or that the documentation is in Slack or whatever. But my question is how to solve this? We can't comment on all the code we have and searching all our documentation sucks. So is there maybe a nice search tool or something we can use?