True enough, and the solutions for lighter-weight schema changes have evolved and weren't always so good, but early stage startups often don't really have the sort of data-weight issues that make this hard, unless they're starting with large data sets already. Even at the scale of small hundreds of millions of rows per table (like the company I'm in charge of the database for) it's not much of an issue.
I had to look it up, apparently adding a column with a non-null default was "fixed" in PG 11 (2018), but with a null default it had been fast for a while:
I had to look it up, apparently adding a column with a non-null default was "fixed" in PG 11 (2018), but with a null default it had been fast for a while:
https://www.depesz.com/2018/04/04/waiting-for-postgresql-11-...