Hey,
I think there is a misunderstanding of soft deletes. Can you explain why and how you link soft deletes and using reindex API?
And also how you link reindex API and zero downtime?
Adding some more context about your goal would help a lot to figure out what you are after (independent from any technology or settings), so that we can maybe go from there.
Side note: Since 7.6.0 the soft deletes setting is deprecated, see Index modules | Elasticsearch Guide [8.1] | Elastic