The Student and Exchange Visitor Program (SEVP) “Study in the States” website blog “Questions from Designated School Officials” has another useful update:
Question: When does withdrawing from a class become a violation of F or M student status?
Answer: Withdrawing from a class is only a violation of F or M status if the withdrawal causes the F or M student to not be enrolled in a full course of study. A student must remain enrolled in a full course of study during each term unless authorized for a reduced course load. An F or M student who withdraws from a class during the term and is enrolled, therefore, in less than a full course of study is out of status.
If your school policy allows the student to withdraw from a course after the semester is already over, the student would have still maintained status by remaining enrolled in a full course of study for the whole semester as defined by the regulations.
8 CFR 214.2(f)(4)
8 CFR 214.2(f)(5)(i)
8 CFR 214.2(f)(6)(i)
8 CFR 214.2(f)(6)(iii)