FWIW, as one of those frequent speakers, it is very unlikely to me that the ACP safe harbor would be able to be retroactively applicable. There is too much potential for abuse. Moreover, the statutory language doesn't modify IRC §401(m), so it would be a rather aggressive stretch for the IRS to apply the new rules to the ACP test as well.
No. Sometimes there is a very clear and simple answer, and that is the case here. I'm pretty sure we have no safe harbor plans that allow voluntary after tax contributions.
I would point out that I have heard mixed opinions about the ACP safe harbor applying or not applying. Like many people I have been listening to many of the industry webinars on the SECURE act, and while I think many of the presenters agree with this, some seem to think the ACP safe harbor would still apply.
So moral is to wait and see what the guidance says?