I have a managed package installed in a subscriber org.
Today I just tried to call some of the packages managed Apex in the subscribers org.
It failed with:
Dependent class is invalid and needs recompilation: Class.TheManagedPackageNamespace.ClassFromTheManagedPackage: line 498, column 13
Method removed by critical update: getFieldsDescribes: 
This is a Sandbox org on Spring '17.
Do I need to create a new version of the managed package for this critical update? What critical update is it?
This might be tricky as the packaging org is still on Winter '17.
The corresponding line from the managed package source:
system.debug(LoggingLevel.Debug, 'X.Y() - getFieldsDescribes:' + Limits.getFieldsDescribes() + '/' + Limits.getLimitFieldsDescribes());
It looks like the Limits method in question was deprecated in Summer '14, but should have kept working for API 30.0 and earlier. Source