I'm the only administrator and I received an email regarding API version upgrade from Salesforce. I have a org that has all apex classes below 20 .Though documentation states API version does not effect the apex classes, visual force pages) but still this concerns me since Salesforce optimizer does say to update the apex class versions any pointer on what everyone's doing about this and does any idea on whether Salesforce provide any plans to extend this retirement time frame? What impact should I be concerned about with this retirement.
Manoj Nambirajan (Dell Technologies) 
Forum Ambassador
for apex classes related.. since its metadata you may need a code scan to do.
atleast for those with a WSDL.. you can go to set up || apex classes , look for classes which has WSDL linked with and check the api version of the classes.
You need to re-deploy apex classes to upgrade api version.
with regards to integrations.. you can look at Login history view under set up
