That's going to be highly dependent on the relay design specifics. The relay will not even start to move until the current drops below the hold-in threshold, and on a typical design, as soon as any gap opens up between the coil armature and the contact lever, the magnetic force will drop quite quickly. I'm not saying what you propose, or have seen, doesn't or can't happen, but delaying the release of the relay is not the same thing as slowing the rate of the release. For high-power applications or those where relay life is paramount, I suppose some testing of this is in order.
Have you had an application where relay life was shortened by a diode clamp? A lot of automotive applications (the only ones I have much experience with) have the diode built into the relay.
Really, do some homework; this is a
really widely discussed and well known matter!
The laziness of "just putting the diode there because it has always worked for me" then requiring evidence for suggesting otherwise while you can find a lot of literature from this is just, well, lazy. Do note I said it's a complex matter where the diode may or may not work, not that it never works. It is a very typical solution; and also a typical design failure. I'm not going to write a book about it for you in a forum post, I already provided far more than the minimum amount of information so you can research for more details including when this does matter and when this does not matter.
Even for a beginner, it's good to know that issues may arise with the diode. Knowledge of something being more complex than it first appears, even if ignoring that issue for time being, prevents surprises like working decades with such circuits, then hear from a younger engineer about the problem for the first time, being unable to believe it exists.
A lot of automotive relays use simple parallel resistor, you can usually order them without coil suppressors, a diode, or a resistor. A simple resistor is fine when the extra power dissipation is not a problem, and it's a cheap single-component solution.
And yes, I have personally seen burned contacts where diodes have been used to suppress the coil. I can't
prove they would have last longer with better coil snubbing, but I don't have to, it's a phenomenon which has been widely discussed and researched.
As a practical matter, the slowing of turn-off is almost never a consideration
It's "almost never" a consideration because of laziness, trusting the luck factor, and doing what everybody "just does". Unsurprisingly, engineers have deadlines to meet as well and as a result, products do fail. Relay contact arcing and either increased contact resistance, or welding permanently-on, are quite common failure modes. The reasons are, substandard relay quality, choosing an improper relay for the load, failure to snub the contacts, or failure to snub the coil. Likely, in some % of the cases, the reason is the last one with the lazy choice of using a diode.
Yet, once you accept that other solutions than the diode exist (i.e., letting go of the confirmation bias), you start seeing those solutions being actually used.
Quick Google result of a typical appnote discussing this showing actual circuit measurement results:
https://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=13C3264_AppNote&DocType=CS&DocLang=EN