Some mobile phones can be muted by turning the device over. I have read that this is also supported by Lumia phones but couldn't find the setting in mine -- is this true? If so, where is the setting?
Unfortunately it doesn't look like there's any way to accomplish this in Windows 10 Mobile. Your best option is to use the Windows Feedback app to let Microsoft know you miss this feature. A quick search for "flip" brings up a number of results that you can upvote.
Windows Phone 8.1 supported this functionality, it could be enabled in Settings → Audio, using the "Flip to silence" option.
Alternatively, one could download the Gestures Beta app, which provided several other similar gestures, like automatically toggling loudspeaker mode (individual gestures were dependent on the presence of specific sensors). After installing the app, it could be found in Settings.
For more information, see the following links: