forum.freeciv.org

Hmm.

Bumbi has finished building Curtain Walls.
Bumbi is building Airport as suggested by the advisor.
...
Year: 1892 CE
...
Year: 1894 CE
...
Bumbi is selling City Walls (obsolete) for 60.
Learned Advanced Flight. Our scientists focus on Photography; goal is Television.
Year: 1896 CE

So yeah. You can make buildings obsolete_by other buildings, but that appears to only actually get checked and the obsolete building sold off when you learn a new tech.

You can make buildings obsolete_by other buildings, but that appears to only actually get checked and the obsolete building sold off when you learn a new tech.

I opened an issue for this -

https://redmine.freeciv.org/issues/2087

Also opened an issue for the bug mentioned earlier about the F6 tech tree nodes showing buildings with negative tech reqs -

https://redmine.freeciv.org/issues/2085

If you make your own builds, you can download those patches and fix your build.