What happens to the current Self-Assessment return system?

If you have signed up for the pilot, you do not need to submit under the current system – with some exceptions as below. If not, you still need to submit one for the tax year as expected.
Pilot scheme exceptions: If you have a declaration for things outside of the initial scope for MTD for ITSA, you will still need to submit a Self-Assessment return in addition to your MTD for ITSA obligations. At the time of writing, examples of these are gains, charitable donations and certain tax reliefs.