No, Gribouille, the information on the wiki is out of date, as Mobian no longer contains the mentioned file or a file with similar details that can be changed. I assume that some changes were recently made to make Mobian closer to Debian Bullseye, but the devs have not provided updated instructions for enabling Mobian unstable. (Perhaps now, with the release of Bullseye, "Mobian unstable" will simply be plain Debian unstable, but I don’t know.)
I am trying to get my pinephone to the point of being a everyday driver, so I need to be able to install some packages from unstable before switching back. The instructions in the wiki for unstable are no longer correct, I want to get unstable working in the correct way rather than using a hack which could leave me with broken important packages. I carried over working unstable settings on my old system board install but now with the new upgrade 3/23gb board I have a fresh install not a rolling upgrade.
The wiki still doesn't have the updated instructions....