I'm pretty sure that I've seen some "how a realist hero rebuilt the kingdom" porn here some time ago though I can't find any trace of it now. Did I imagine it or was this content removed?

I can't find anything obvious. No hits for realistic or realist as tags. If you remember any specific character names, try searching for them with status:deleted.

I searched the tags for all 4 variations of the title I could think of and didn't get anything. Gelbooru & danbooru both have results under its romanized title though (Genjitsu Shugi Yūsha no Ōkoku Saikenki)

Obligatory 'help me find this' thread link

Are you sure it was on this site? You can also try searching for status:deleted alongside the title.