This is relevant:
<https://forum.httrack.com/readmsg/22939/index.html>
Inside, someone suggested to download everything, then use a tool to find
files containing your phrase, delete these and set httrack not to re-download
removed files (if you plan to run the mirroring again in the future).. 

<https://youtu.be/315aLh4iX-8?t=64> (pause after opening) - it says that it is
not possible (in 2020) without development skills.