Search seems to rely on 6.8.3
From discussions with Microsoft (@dkodeih) it seems the Search Service seems to rely on version 6.8.3. As 6.8.8 is the latest, it is strongly advised (boarding essential) that the search service uses 6.8.8 as a minimum at the time of writing this. Ideally it would be using 7.5.2 or 7.6.2.
For production it is best practise to always use the previous minor and latest patch for the latest major. For example, given the below versions, I would recommend OSDU runs on 7.5.2 in production:
7.6.2
7.6.1
7.6.0
7.5.2
7.5.1
7.5.0
This way you benefit from all the latest patches, bug fixes, performance increases and new features. I would also be wary of our end of life policy. 6.8.x is supported until November 20th 2020.
From conversations, it seems that the search service is tied to 6.8.3 as it doesn't work with later versions. From briefly checking the code I couldn't find how/why this is the case, however there are not any breaking changes between 6.8.3 or 6.8.8.
The reason that the service should use 6.8.8 as a minimum as the later patches involve a lot of security patches and bug fixes making it a more stable and secure solution.
Similar Issue: indexer-service#2 (moved)