I installed MacOS 15.4 Beta (24E5222f) today.
When I started Safari for the first time, I noticed that de develop menu was missing.
I tried reenabling it in the settings, but nothing worked.
I knew, it somehow had to work.
So the trick was to look at the settings via CLI:
defaults read com.apple.Safari
There is an entry:
com.apple.Safari IncludeDevelopMenu 0
So I changed that by entering:
defaults write com.apple.Safari IncludeDevelopMenu 1
Et voilĂ :
Starting Safari again, the Develop menu was back.
Cheers