I know this isn't really what you're asking, but from the health plan side I view this as a red herring. The deemed 125 compensation issue addresses employers that condition the cash option on proof of other coverage, which then puts you outside the cafeteria plan safe harbor from constructive receipt. I almost never see that approach in practice.
For one, the ACA employer mandate rules require that employees have an effective opportunity to decline the employer’s offer of coverage. If the plan does not follow the federal poverty line safe harbor for determining affordability, there is an argument that the employee does not have the effective opportunity to decline coverage if they must show proof of other coverage.
Furthermore, requiring proof of other coverage raises potential state wage withholding law violations. Employers generally cannot withhold amounts from an employee’s paycheck (including premium contributions) without the employee’s authorization. Requiring proof of other coverage to decline the employer’s offer of coverage effectively compels employees without other coverage to pay the employee-share of the premium.
It is very common for employers to require proof of other coverage to receive the opt-out credit, but that's a different situation that doesn't create deemed 125 compensation. That's an affordability requirement to meet the ACA employer mandate definition of an "eligible opt-out arrangement."
Treas. Reg. § 54.4980H-4(b)(1):
(b) Offer of coverage—(1) In general. An applicable large employer member will not be treated as having made an offer of coverage to a full-time employee for a plan year if the employee does not have an effective opportunity to elect to enroll in the coverage at least once with respect to the plan year, or does not have an effective opportunity to decline to enroll if the coverage offered does not provide minimum value or requires an employee contribution for any calendar month of more than 9.5 percent of a monthly amount determined as the federal poverty line for a single individual for the applicable calendar year, divided by 12. For this purpose, the applicable federal poverty line is the federal poverty line for the 48 contiguous states and the District of Columbia. Whether an employee has an effective opportunity to enroll or to decline to enroll is determined based on all the relevant facts and circumstances, including adequacy of notice of the availability of the offer of coverage, the period of time during which acceptance of the offer of coverage may be made, and any other conditions on the offer. An employee's election of coverage from a prior year that continues for the next plan year unless the employee affirmatively elects to opt out of the plan constitutes an offer of coverage for purposes of section 4980H.