The code change there in hrm ticket prevents buying when an building is already obsolete during the turn. It can't do anything for the typical case that building has been bought, gets obsolete during the turn change, and then bought shields count toward new target. So the rule would not be really consistent.
From https://www.hostedredmine.com/issues/921933 by Ilkka Virta: Prevent buying an obsolete building.
As part of #43200, this has also been posted to http://forum.freeciv.org/f/viewtopic.php?f=14&t=92066