This advice grew out of the days when database platforms cost enormous amounts of money. It is still as true as it ever was, but since then the biggest pressure

This advice grew out of the days when database platforms cost enormous amounts of money. It is still as true as it ever was, but since then the biggest pressure that lead to it no longer applies. Back before PG and others were viable competitors to the big commercial databases, about the only leverage customers had against aggressive sales people was the threat to switch to a competitor. So people would preserve that as a credible threat by not using proprietary SQL extensions. No vampiric salesdroid from Postgres is going to knock on your door demanding 8 figures. Now, there may be other reasons you want to switch away from it, or you may want to support a variety of DBs in whatever you're building, or some other reason. So it is good to be aware of. But shying away from SQL extensions is usually no longer a matter of budgetary apocalypse.