Analysis shall include assessment of the impact of the release on the
customer. I am looking for what all the information one need to cover as
part of this report post release?
This part of the Release and deployment management process (clause 9.3 in the ISO 20000-1) is actually asking from you to check the efficiency of the implementation. Namely, releases (particularly less successful ones) can be source of new errors i.e. incidents. By analyzing:
a) impact - this will tell you how big is the "damage", and
b) incidents itself - this will give you insight information what went wrong, who is responsible, why...etc.
you can get valuable information about further proceeding. Often such analysis results in improvement initiatives.
So, what you should do is analyze newly created incident (i.e. incidents which occur as a result of the release) and document your findings. Result can be e.g. do-nothing (e.g. when impact of the new incident is not significant and customer agree with it - less likely but sometimes it could happen) or new improvement initiative (e.g. when you are aware what is wrong and now you have to remove cause of the incident).
These two articles will give you general view on Release and deployment management:
ITIL Release and Deployment Management Part I – General principles and service testing - https://advisera.com/20000academy/blog/2014/01/15/itil-release-deployment-management-part-general-principles-service-testing/
ITIL Release and Deployment Management Part 2 – deployment methods and early life support - https://advisera.com/20000academy/blog/2014/01/28/itil-release-deployment-management-part-2-deployment-methods-early-life-support/