Method removed by critical update: getFieldsDescribes
There's one available in my org that seems relevant:
Remove Deprecated Limits Method
Update Name
Remove Deprecated Limits MethodAuto-Activation Date
Fri May 19 00:00:00 GMT 2017Impact Details
Removed the following static Apex methods from the Limits class.
- getChildRelationshipsDescribes() and getLimitChildRelationshipsDescribes()
- getFieldsDescribes() and getLimitFieldsDescribes()
- getFieldSetsDescribes() and getLimitFieldSetsDescribes()
- getPicklistDescribes() and getLimitPicklistDescribes()
- getRecordTypesDescribes() and getLimitRecordTypesDescribes()
- getScriptStatements() and getLimitScriptStatements()
Can't find anything related to it in the Release Notes though, nor can I answer your other questions. Still, this might help investigate.
UPDATE: Salesforce have amended the release notes to include Removal of Deprecated Apex Limits Methods (Critical Update)
These methods were deprecated in Summer 14. You've had almost three years to remove those methods from your code. You should remove this code from your package as soon as possible. This critical update was to bring this to your attention, because salesforce plans on removing this method entirely, but can't do so until everyone's stopped using it. In the interim, your subscribers can turn off the critical update (for now), but you need to fix this before Summer '17.