Close, but not quite. If you google ActivityWatch you can see the application I am developing. It currently displays how much time you spent in different applications, websites and more but we have been considering adding the feature of notifying you if you spend too much time in a specific application or website. While that is not identical to how FocusMe works, it should be good enough for many users use-cases.
The issue is that the program in the PureOS operating system which is responsible for letting applications display their windows on the screen keeps it private from every other program what windows are being shown so it is currently impossible for other applications to track that (which is good from a security perspective, but for us who use applications which need this data it is problematic).
So I have been pitching for a standard way for applications to fetch such information and that proposal can be found on this link where it’s possible to add a vote if you have an account on GitHub.