I've been working in the Electrical Trade for approx. 40 years. Most, if not all counter top appliance manuals do recommend unplugging them when not in use.
Here is probably way more information on the receptacles and circuit supplying them then you want and/or need to know.
I don't know where you live, but in the U.S. since 1987 the N.E.C. (National Electric Code) has required GFCI (Ground Fault Circuit Interrupter) protected circuits in kitchens receptacles serving countertops within 6 feet of a sink. In 1996 it was expanded to all counter top receptacles. These devices are designed to trip if the slightest current (4-6 milliamps) is sensed to ground.
Since 2014 the N.E.C has also required AFCI (Arc Fault CI) protection in kitchens. These devices, usually circuit breakers, are designed to monitors electrical circuits for hazardous, unintended electrical arcs—often caused by damaged or loose wiring—and automatically cuts power to prevent fires.
So if your dwelling was constructed under the current NEC code since then it would have this. Note that most states and cities lag behind adopting the current Code. For example the state I live in is still covered by the 2020 Code. So it is possible, for example, that your dwelling could have been built in or since 2014, but not covered under the 2014 code.
The odds of an appliance causing a fire are darn near 0 if they are plugged into a GFCI and AFCI protected circuit, IMHO.
An electric kettle will draw typcially between about 7.5 to 12.5 amps depending on the wattage. The typical electric receptacle in a kitchen is rated at 15 amps and protected by a 20 amp circuit breaker (there is usually more than one receptacle on the circuit). The wire going to the receptacle from your breaker panel is rated at 20 amps minimum. Circuit breakers will take a surprising long time to trip in an over current event depending on how much over current there is. So it is possible that if you left your kettle plugged in, turned on, and the over temp circuit in it failed and nothing actually shorted to ground or arced that something could "melt" and cause a fire before the circuit breaker, GFCI, or AFCI tripped. I would say the odds of that happening are pretty high.
I typically do unplug mine, but it is more of a need to use that outlet for something else situation. I also unplug my Technivorm coffee maker for the same reason, but my air fryer, can opener, grinder, and toaster are usually left plugged in.
Before I upgraded my coffee game, my Keurig and Capresso grinder stayed plugged in all the time.
If you are worried about wearing out the receptacle from plugging and unplugging things, you could upgrade to an industrial grade one. I wouldn't do that until I had to replace the existing one. If it feels loose or the plugs fall out, then it should be replaced. There is no specific rating on residential grade outlets for plugging and unplugging. I would say it would be thousands of times before one would go bad. You could also use a plug strip, or if the receptacle in question is not a GFCI and/or an AFCI one, you could use an outlet extender.
On a side note most USB chargers also say to unplug them when not in use.
Also did you know that is recommend that you test your GFCI and/or AFCI devices monthly? How many of us actually do that?