In FontBook, how do I re-enable the "...Remove Font Family?" warning?

font-book

Tested in macOS Catalina, with Font Book closed, from Terminal:

defaults delete com.apple.FontBook FBRemoveFontsFromLibrary

Allows this dialog box to show again:

enter image description here

Related

how to open multiple vlc interface (window) in xubuntu 14.04 lts
Revert from dark nimbus theme to default in netbeans
vlc plays all videos in same window
Screen flickering on mouse move (Ubuntu 20/21, Lenovo IdeaPad 5i Pro)
Is there a way to permanently rename a FTDI TTL to USB Serial Converter
How do you develop an app on Ubuntu with a simple GUI? [closed]
Canon EOS 600D as Webcam
How do I make Synaptics touchpad feel more like on Windows?
Nginx and a per server's user setting
Upgrade from Ubuntu 20.04 LTS to Ubuntu 21.10
kill app in a given timeframe, bash script
How can I encrypt a file with gpg non-interactively?