According to git log, Nov 8th 2021 was the day Jsfxr Pro was started. The original free tool sfxr.me had been online for many years and gets thousands of visitors each month.
I started Jsfxr Pro by building a simple landing page explaining the new features, and an email form so that people could sign up to be notified of the release. I linked it from the popular free tool and waited to see how many people would sign up.