We use an issue tracking system like Jira, Trello, Asana, etc and each "ticket" is a unique identifier followed by a brief description. You can add all other s

We use an issue tracking system like Jira, Trello, Asana, etc and each "ticket" is a unique identifier followed by a brief description. You can add all other sorts of labels, descriptions, etc to better map to the requirements you get. Next, all git branches are named the exact same way as the corresponding ticket. Unit tests are created under the same branch. After getting PR'd in, the code and unit tests can always be matched up to the ticket and therefore the requirement. For us, this system is good enough to replace the usual plethora of documentation the military requires. It does require strict following that can take extra time sometimes, but all devs on my team prefer it to writing more robust documentation. Another useful tool to use in conjunction to the above is running code coverage on each branch to ensure you don't have new code coming in that is not covered by unit tests.