Safari 17.3 - Where did Menu Item Develop: Disable Java go?

Safari stopped supporting NPAPI (and similar types) plugins in version 12. But maybe someone out were wrote a Java Applet to Webassembly conversion tool.

If you're asking about Disable JavaScript, that option is now in Web Inspector in the Device Settings popover in the upper-left corner.

It is a per-tab setting, instead of a global setting, so you can disable JavaScript only in the current tab leaving all others unaffected.