In MacOS Catalina 10.15 on Safari, How do you disable Command-I from composing a new Mail message?
macOS has a built in method for changing/adding keyboard shortcuts


Open System Preferences
Go to Keyboard > Shortcuts > App Shortcuts
Click the "+" button and select Safari from the Application drop down menu
In the Menu Title box enter the EXACT wording of that menu selection "Email This Page" (it must be exact, capitalization, spaces, etc)
Assign is a keyboard shortcut you would never use, like Shift-Command-Option-Ctrl-I
Click Add and close system preferences


Note now that the Command-I in that menu has been changed to whatever you set it to, allowing you to set Command-I to something else, or just leaving it that way so you won't accidentally use it.