"Therefore I don't think the elective deferral limit applies to the after-tax contributions."

Everything has a limit. That's where we started. That's where we've gone. That's where we still are. The elective deferral has a limit, and it is a subset of the total limit. For clarity, let's review: we've identified that this discussion is in reference to the "after-tax basis funds optionally put into the (trad) 401(k) plan, so that later withdrawals of these amount are taxed on growth only, or so that a nontaxable backdoor conversion to Roth can be done."

"Now is the limit per person or per plan?"

Yes:) Plans have different provisions and can have their specific limits, and the IRS maintains a personal limit for the tax year. Your initial question for your taxpayer was having two completely separate employers and being a W2 earner for each job. Their plan(s) have provisions, which can differ.

"Can my client, who switched employers and has reached the $69K limit from the first employer, contribute after-tax 401K with the new employer?"

I found you this article to read:

From: https://www.bankrate.com/retirement/after-tax-401k/

"You can still have an after-tax 401(k) even after you’ve maxed out your traditional or Roth 401(k) contributions for the year, if your employer allows it."

Just because Congress and the IRS have a bunch of provisions, the plan can not allow or offer them, such as after-tax additions, loans, or hardship withdrawals. For all you know, their new employer doesn't even allow it. And since the employee has reached their annual max, it's moot.

"Expands the 401(k) contribution maximum. If you’re looking to put away more into a tax-advantaged retirement account, the after-tax 401(k) lets you do it. You can contribute up to $69,000 (in 2024) annually or $76,500, if you’re at least 50 years old, including any employer matching funds."

The bold italics are my emphasis.

"Another excerpt from the IRS link. I think the answer is yes.

"Remember that annual contributions to all of your accounts maintained by one employer (and any related employer) - this includes elective deferrals, employee contributions, employer matching and discretionary contributions and allocations of forfeitures, to your accounts, but not including catch-up contributions - may not exceed the lesser of 100% of your compensation or $69,000 for 2024"

From: https://money.usnews.com/money/retirement/401ks/articles/how-to-make-after-tax-401k-contributions

Key Takeaways