Apple revised the support page for Apple Pencil with an additional safety guideline. The revision includes the interference when you charge your Apple Pencil on top of the right side of your iPad Pro. This only applies to the second generation Apple Pencil, and the latest generation iPad Pro models.
According to the revised footnote that Apple added, it states that if you’re charging your Apple Pencil (2nd generation) on your 2018 iPad Pro, and your car key fob is nearby, the magnet on your iPad supplying the charge to your Apple Pencil will interfere with the radio signals. This can eventually cause the key fob to forbid from unlocking your car, unless you put your iPad Pro at least 5 feet away from the key fob, or put your Apple Pencil in a separate case.
In addition, you shouldn’t put your Apple Pencil on ANY side of your iPad Pro when it’s next to your car keys. The best idea is to put it in a sleeve or in a holder of the iPad Pro case (Most iPad Pro cases have a Pencil holder, but the case must not have magnets next to them). Otherwise, you have to wait for your Apple Pencil to fully charge itself to 100% before you touch your key fob.
To conclude, all of the above is the information about radio frequency interference between the 2018 iPad Pro and the Apple Pencil. So next time, don’t charge your Apple Pencil before you unlock your car!