Requirements tools for PCB design?
I did a co-op at a company where I had to design a PCB. This company used Atlassian JIRA, an issue tracker used primarily by software engineers. However, I also used it to track my physical board requirements and such. It integrates with Github or BitBucket, both of which provide version control.

I would say your needs also depend on how many people are involved. JIRA is suitable if you have an entire team working on a project. If it's just you, someone else on the team, and the clients, then I would suggest Github or Bitbucket alone. At the very least, version control allows you to upload your requirements, schematics, or gerbers, update them, and track them. Everything would be shared amongst you and the client. It is definitely more efficient than emailing back and forth.

If you are not familiar with version control, I recommend a google search on what version control is, and also how to use version control with hardware projects. Good luck!