The customer want to know is periodically full reindex recommended  or not? There is a  contradiction between Document 1 and Document 2.

Currently Jira should ensure consistencies, and no need to do periodically reindex for concerning inconsistency problem.
However, how about fragmentation problem mentioned  in Document 1. 
If it is not a common case, can we remove it from the document?

Document 1

It is recommended in https://confluence.atlassian.com/adminjiraserver/search-indexing-938847710.html

Over time, your instance’s disk becomes fragmented from normal use, which slows down your instance. Only the full re-index can address this. This means that, regardless of whether you’re running a single- or multi-node instance, you should run the full re-index to address fragmentation periodically (for example, weekly or monthly).

Document 2

However , it is saying Periodic full reindex in Jira Data Center is not required in  https://confluence.atlassian.com/jirakb/periodic-full-reindex-in-jira-data-center-is-not-required-1188764401.html
Also, according to community page,  re-indexing may not be a part of a scheduled maintenance routine.