Not sure what you mean. Previously CTEs were always materialized (i.e. stashed into a buffer, somewhere), in PostgreSQL 12 it's optional and disabled by default for various cases that don't require it. The problem with materialization is that it prevents various optimizations (e.g. using index for a condition from outside the CTE).

And while some devs might be aware of the materialization (and possibly even leveraged it intentionally, to block some optimizer behavior) many did not - it's widely used as if it was just a nice way to name the subqueries.