Is it safe to disable baloo_file_extractor in KDE Plasma 5?

It's safe to use balooctl disable.

You need to be careful while installing software. Pay attention to what's being installed and to what's being pulled in by way of dependencies. For example, you may have installed dolphin, the file manager. That would pull in libkf5baloo5 and libkf5baloowidgets5 among other things. gwenview, an image viewer, would pull in libkf5baloo5.

If you're interested in chasing things down, run apt rdepends libkf5baloo5 and apt rdepends libkf5baloowidgets5 and examine the output for software packages you may have on your system.

Under normal circumstances in Kubuntu 18.04, which includes baloo by default, use of resources is minimal after the initial indexing process. RAM and CPU usage are not normally a concern. But there are reports of baloo running amok even on pure Kubuntu! I use it without problems for indexing both filenames and file contents.