With the introduction of iOS 13 and the launch of the iPhone 11, iPhone 11 Pro and Pro Max the way that you clear the RAM on the devices has changed.
Zollotech have put together a great video which shows us how to clear the RAM on your iPhone without having to do a full reset on the device.
As we can see from the video, you will need to turn on Assistive Touch on your device in order for this to work. This can be done by going to Settings > Accessibility > Touch > Assistive Touch and turn it on.
Once this is done you need to press the volume up, then the volume down and then press and hold the power button and the release it. You then need to press and hold the Assistive Touch Home Button, then enter you passcode and your RAM has been cleared.
Source & Image Credit: Zollotech