KLM Flex ticket refund window
 

Am looking at a KLM flex fare.
T&C say refundable any time before departure - do KLM stand by that ?
(Online I found several mentions of "at least 3 (or 4) hours before departure" )

No, they don't stand by that.

Flex tickets (on shorthaul anyway) don't allow refunds after a no-show; only for a rebooking.

If you're working on a strategy whereby you will only be in a position to attempt cancelling a flight in the dying minutes before checkin closes - at a time when you are presumably under some pressure/stress and may not be able to contact the correct person on the correct number in sufficent time - I would advise you to consider if there aren't any other better options for whatever kind of fall-back position you are working on.

I'm a great advocate in planning pessimistically. But instead of buying unnecessarily expensive (Flex is expensive) tickets, I usually have a different strategy, involving cheaper, intentional throw-away tickets. I'd book a backup throwaway ticket that would assist me in case of whatever misconnect or other worry you are may be attempting to address. Such a throwaway ticket, which although not refundable, can usually have the taxes refunded. If booked sufficiently cleverly and long enough in advance - and on any cheap carrier - it's a better insurance policy than betting on being able to put yourself in a position where you have enough time to cancel this unnecessarily expensive Flex ticket.

Have you had an experience where KL actually denied a refund claim made before departure?
Not talking about no-shows since it's not what OP is asking about.

I've had to cancel tickets countless times (granted, with AF not KL) when it became clear I would not make it on time to the gate. Always during check-in period, before gate close cutoff. Never had any issue.

No, but as I understand it, the OP is asking for when, precisely, it is no longer possible to cancel and claim a refund.

They are clearly not talking about the usual position whereby a person can cancel months, weeks or even days before departure without worry.

I do not know when, exactly, it becomes impossible to cancel "in time". Is it when OLCI closes? Is it when the airport check-in closes? Is it when the gate agent closes the flight? Is it when the gate agent hands the manifest to the captain? Is it when the aircraft door closes? Is it at pushback? Is it when the craft reaches the active runway? Is it at wheels up?

I'm pretty sure that for most, if not all, of these timepoints, it would not be possible to simply cancel the ticket online or in the app, and whether and if you can get through to an agent on the phone who has the authority/ability to cancel the ticket at any of these times is also a bit of a worry.

My advice to the OP is not to plan a course of action where they literally will not know whether/if they want to cancel a flight until so close to departure.

"Before departure" is not a precise statement; you cannot derive from this at which specific moment in time cancellation may become impossible.

OP is asking whether KL honors the option to cancel before departure for flex ticket.

TA and PSL often mentioned "we are before departure so you can cancel", so I would not worry about it too much since it's also what's written on the T&C.

I always played it safe and canceled while airport check-in was still open, but I am yet to see any data point that would suggest that AF or KL don't stand by this written rule.

That's why I am asking why you write that they don't stand by it.

You use the phrase "played it safe"; that's also my advice to the OP.

"Before departure" is an imprecise phrase. And what the OP has written seems to indicate they're interested in waiting until beyond 3-4 hours before departure. The OP's interpretation of "before departure" here might end up very differently than what KLM will actually impose; waiting until the absolute last second is always risky, especially when it's not 100% clear when that "last second" actually occurs.

I'm advising against whatever plan the OP is focusing on as it apparently puts them in a position where they won't be able to cancel until so close to departure that it might not actually be feasible to cancel the ticket in time.

It's not. STD is a well-defined term, and again I've never had (nor heard of) any case where AF denied refund because the request was before STD, but after STD-3, STD-4, or whatever.

Your cost-optimization strategy is sound, but no need to worry OP, who might be reading us with a flex ticket on hands, about a hypothetic breach of contract unless someone reports it actually happened.

Were you able to do this online or had to call in? Does the online refund option/button remain available until STD?

Mostly on the phone, since my SOP is to ask if rebooking is possible at an acceptable rate.

...and reporting back.
Had to cancel about two and a half hours before departure - went very smoothly. Cancelled via the app, got a refund mail, was refunded in full within days.

Generally yes, but for some tickets that is absolutely not the case.
The flex ticket I purchased was ~ 100-150 Eur more than the cheapest economy.