Sure it is! As a thought experiment consider evil dba whose job it is to crash your application or make it do wrong things just by manipulating the data in the DB but following the constraints. Totally trivial, right? So data integrity is at all times the responsibility of both the app and the db. And the set of constraints you can enforce with the app will always be a superset of what can be enforced by the db. And for some cases (usually when the db is private to the application) it's easier to build that logic into the app and for other cases (multiple apps sharing a db) it's easier to build that logic into the db.
I get where you're leading but I don't think it follows. FK constraints are an optional nicety with tradeoffs rather than something fundamental -- cascade and set null are footguns (and business logic which shouldn't live in the db), and no action exists to catch bugs in your code. If your app isn't getting errors from the db saying it's trying to delete stuff with references then you could, in theory, turn off the constraints without any need to change the code. And if a relational database simply didn't have FK constraints at all, such as vitess/planetscale, you can still maintain data integrity.
What I've done in the past with prod dbs that lack FK constraints is add them back for development, testing, and CI.