The only reason why someone would want to disable file indexing entirely is because they don't want their files indexed. This implies that if their files are already index, they would like the index deleted. Ergo, disabling inexing in the KCM should delete the index too.
Marking assigned as Nate has already created a patch for it, have requested on the Phab diff that it be moved to Gitlab.
A possibly relevant merge request was started @ https://invent.kde.org/plasma/plasma-desktop/-/merge_requests/427
A possibly relevant merge request was started @ https://invent.kde.org/plasma/plasma-desktop/-/merge_requests/432
Git commit f668bcda506dbf93893c4907725f286b9caaa0b6 by Nate Graham.
Committed on 23/04/2021 at 00:44.
Pushed by ngraham into branch 'master'.
[kcms/baloo] Offer to delete index after indexing is disabled
When the user disables indexing, the KCM now offers to remove the saved
index data. This may be useful for privacy or security purposes.
M +22 -0 kcms/baloo/kcm.cpp
M +3 -0 kcms/baloo/kcm.h
M +19 -0 kcms/baloo/package/contents/ui/main.qml