It says

As these installation methods are used for the development of Home Assistant, it will still be technically possible to update them. We still would recommend migrating to a supported method, but that’s your choice.

And then towards the end:

Will the developer documentation on these things remain?
Yes, those will remain. The developer documentation for running Home Assistant’s Core Python application directly in a Python virtual environment will remain. This is how we develop. This proposal is about removing end-user documentation and support.

How I read it is that these methods are actively used for development so will still be maintained and updated, including developer documentation because developers will continue to need to use these methods.