You have a very city-centric view, I think.

Here, in Canada, some cities allow the keeping of chickens. These are >1M population cities (not greater metro pop., but actual cities).

In suburban areas, this is more wide spread, and in rural communities? Now legal where before it was not.

This is not just for eggs, but butchering too.

(There are limits on numbers per household...).

So it has already happened where I live.

You may not be aware, but you don't have to butcher the animals yourself. Here, your local butcher will, just as with any game animal.

Allowing does not means that's doable: chickens need to eat, as any living being, if you can spare a bit of precious (in such dense areas) space to rise enough some chickens but you can't nourish them except buying foods from local grocery store... You can just keep them, perhaps buying some crappy food from them at the same price of eggs and already butchered meat...

Here (France) I've already seen small 5kg sac of cereals "for poultry" at 5 EUR, while a cooked, ready to eat, chicken is about 4 EUG/kg and eggs can be found around 10 cent per egg...

For pig is even worse: I can found pig meat ready to make sausages at home (just grinding, adding salt around 15g per kg of meat, a bit of pepper, anything else you like) less than 2€/kg...

Also the breeds are entirely different between chickens that produce eggs and broilers we consume.

Also, a whole pig is pretty huge investment. They are not small animals if allowed to be grown up.

Ok. Do you think fewer people will live in cities in 100 years?

Current population trends (declining), along with remote works, says perhaps.

But that's not relevant, because most cities I know in Canada, have lots with space for chickens.

Neighborhoods may also have communal chicken coops, just like the garden plots they have currently.