I'm looking for a story that I read online (probably a web-series with a separate website) where the core setting was a somewhat dystopian future where a drug for effective anti-aging has been found.
Different regions had different solutions to the obvious resource constraints caused by this invention. The protagonists were from "Europe" where the solution was that everybody had access to immortality as the drug was included in the water supply, but because of that had a total population control - if you chose to have a child, you'd be injected with a chemical that would kill you in 20 years, so that the population wouldn't grow; other places had unrestricted growth of children but restricted the availability of that drug so that the poor people couldn't afford to live forever.
I recall the a protagonist who wanted to disrupt the system (and likely save his/her immortal life because of getting the "Terminator" injection); and that the action happened mostly in very very high arcology towers named after major current cities e.g. Barcelona was on the border and location of some activity.