Back in the day nothing...they decided to start enforcing it last summer when contract negotiations broke down.

“First, management issued "warning letters" to pilots who have not credited 70 hours in a month. While it's true that the CBA states that a pilot must credit 70 hours by the end of the month, management has not enforced this provision for many years, and told us at the bargaining table that it had no intent to enforce the provision. However, despite the clear past practice that allows pilots to credit less than 70 hours in a month, management has threatened to discipline such pilots.”

I don’t know anyone who has tried since so who knows what they would do now.