There used to be thousands of entries under the "stealth sex" tag, it was one of my favorites. Now it's less than 100. For what possible reason would mods do this? How does that help make the site better in any way?

There's about 2.8k posts (37 pages) with the stealth_sex tag.

If you're getting so few results I'm going to assume you're doing something along the lines of searching stealth sex instead of stealth_sex - words in tags should be separated with underscores instead of spaces, otherwise you're searching for posts that contain two separate tags, stealth and sex.