Identity Service version

If referring to the OOTB Identity Service in K2 it will be linked to what version of K2 your running. If you on the latest then you have the latest.

HTH
Vernon

Does page install the latest identity service for a given K2 version (e.g 5.5) regardless what CU is installed?
e.g. If we are on 5.5 CU3, how do I know we have the latest Identity Service for 5.5?

https://community.nintex.com/t5/Technical-Issues/Identity-Synchronization-and-Caching/ta-p/124719

I think that if you have K2 Five 5.5 and the add-on was installed right after installation, and only then you installed some cumulative update and/or fix pack, you have the latest version.
I don't think you need to download the version from that link and install again to get something newer.
There was just a fix pack released for K2 Five 5.5 that addresses something related to that add-on.
https://community.nintex.com/t5/K2-Five-Cumulative-Updates-and/Code-Fix-SyncEngine-Operation-not-working-for-AAD/ta-p/226988

Yes, that installer merely enables the feature, as far as code base of the feature goes that would be coupled to being on the latest CU and FP.