FontForge disappeared from Mac

How did this happen, ...

Impossible to determine given the information provided.

That Mac has not been affected by any non-Apple "cleaning" / "security" / "anti-virus" garbage by any chance, has it? Never, meaning at present or at any time in the past? Needless to say, if you are using anything at all in that broad category of utterly useless garbage, don't. A lot of things won't work if you do.

Inexplicable app disappearance is disconcerting at best, since it calls into question what else may have spontaneously disappeared, perhaps without your notice.

Time Machine conveys the ability to "undo" such mysterious disappearances: Back up your files with Time Machine on Mac - Apple Support, but of course such mysteries will continue until the cause is identified and addressed.

You don't mention whether you are on an Intel or Apple Silicon Mac. FontForge requires macOS 10.15 or later, and depends on Rosetta 2 (and preferably Ventura+) if you are on an Apple Silicon Mac. It does require a compatible version of XQuartz for your macOS operating system.

There is nothing in macOS that would automatically remove FontForge from your Mac. That would be a manual process.