If an employee willingly wants to switch from full-time status to part-time, is the employer still responsible for providing health insurance benefits because they were hired as full-time?
The requirement to provide coverage is based on current full-time status and length of employment, not full-time status at time of hiring. If an employee switches to part-time they no longer have to be offered coverage starting on a specific date, that specific date depends on complex factors related to measurement periods.
Once a full-time employee is in "a stability period" (a type of measurement period), they must generally be offered coverage until the end of the calendar year regardless of their current hours worked (if they are employed, they must be offered coverage). With that said, the law allows for extra flexibility, resulting in shorter periods in which coverage has to be offered (6 months is the lowest right now, 3 was pushed for but rejected).
So, while full-time status is based on hours worked, the specific rules for measurement periods complicate things considerably in terms of how long coverage has to be offered and to whom.
One important note is that if you move to part-time, and thus your income changes, you may be able to be exempted from an employer plan due to affordability..
If you lose coverage (for any reason other than non-payment or quitting the plan, including an affordability exemption) you have a 60 day Special Enrollment Period to switch to a Marketplace plan (and a 30 day Special Enrollment period to switch to an employer plan, for those in the opposite situation).
If your employer does cancel your coverage because of you moved to part-time (after the applicable period), then this triggers special enrollment in healthcare.gov. If you choose to quit the plan, or are dropped for non-payment, it doesn't.
This answer may be complex, but it is never-the-less the correct answer. We had previously misunderstood details related to full-time status, but this current answer should be correct despite being rather general. Please see the links on the page for the nitty gritty details.
Learn more about the Employer Mandate.