Is that a regional thing? In my part of Europe virtually 60-70+ year old house comes standard with it. But the fact that English lacks a word for “soupirail” (or that Wikipedia only has a page in French and Dutch) leads me to believe it might not be a very widespread practice outside of Western Europe.
It often gets repurposed as an air vent, for general ventilation and/or as a clothes dryer exhaust. I rarely see them permanently blocked out.
My cheap scale will not work with my freshly recharged amazon basics AAAs. Apparently those do not hold up well enough under load. And of course any set of batteries that does work, discharges over a few months.
So I just bought a cheap mechanical scale (not from Amazon) and I eyeball it for weights under 50g. Good enough and dead reliable.