How to find API calls using deprecated versions of the Salesforce API
Salesforce just announced that on June 1 this year, they'll deprecate API versions 21.0 through 30.0
If you have Event Monitoring enabled in your org, you can use this simple script to find API calls using older API versions.
The script is configurable, so you can specify the max API version you want to find calls for.
If you don't have Event Monitoring: RIP