Disable the physical Silent Mode switch on iPhone SE 2020? I keep missing calls because the Silent Mode hardware toggle (outside left edge) keeps flipping inadvertently.
 Is there a way to disable that switch, to prevent my iPhone SE from ever going into silent mode?
When I need silence, I can simply shutdown the iPhone.
No

 Is there a way to disable that switch, to prevent my iPhone SE from ever going into silent mode?

There is no way to disable the working of the physical silent switch.
Flash for alerts
If you keep missing calls due to the switch inadvertently getting flipped, you may consider enabling LED Flash for Alerts, a setting that flashes the iPhone camera's flashlight for alerts.
This may help with catching your attention if the iPhone is on silent mode either intentionally or inadvertently.
In iOS 13, the setting can be enabled by going to Settings app → Accessibility → Audio/Visual → LED Flash for Alerts.

Flash only in Silent Mode
While this setting has been around for a long while in iOS, an interesting enhancement to the setting introduced in iOS 13 is the ability to have your iPhone's LED flash only when it's in silent mode.