I realize a similar question to this has been asked, but it's not the same. The answers in that question are for older versions of MacOS. I'm asking this question specifically for MacOS Catalina 10.15.
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 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.
The only solution that I've found to work for Safari is to change the default mail handler app.
I set mine to Safari itself, which isn't quite perfect but at least it means that Cmd+I (when not trapped by a handler for italics) doesn't open Mail.
As detailed in the top answer of Set default mail client in macOS without adding an email account? you can use SwiftDefaultApps to change the default mail app, as illustrated here ↓
You can remap Command+I to a different action in Safari that is less annoying than composing a new email. For example, I've set mine to open the share sheet which can be easily escaped out of.
Go to System Settings > Keyboard > Keyboard Shortcuts > App Shortcuts and map Command+I in Safari to open "Share...":
Or you can run the following shell command and then relaunch Safari:
defaults write com.apple.Safari NSUserKeyEquivalents -dict-add 'Share...' '@i'
Add these to your shortcuts, and try restarting your computer. It should work on Catalina.
EDIT: It seems to be a bug that this method won't actually work on Safari, even on macOS 11 (unless they fix the bug before it's officially released).