Using the bundle, the necessary information: which module pollers have been updated in the bundle release is completely missing.-> This really is a good question , I cant remember but then I guess you will have to unpack the bundle (unzip) to check the module version in it. If I am not wrong the bundle would consist of a readme as well in it.
So how should a customer know if this upgrade is necessary / recommended for them... -> Totally agree with you, this is little time consuming but information can be found on it through various means as I mentioned in my previous comment.
Ironically the Customer Portal Releasnotes Link to Additional Polling Engine points to the normal NPM Release Notes and there is no single word about Additional Pollers -> Solarwinds Support Ticket
If you find an easy approach to find the details you did mention (once support comes back to you), please do post the details here. Thanks
