You might receive a warning from Salesforce regarding the deprecation of older versions of the API, such as this article: Salesforce Platform API Versions 21.0 through 30.0 Retirement.
From Salesforce's documentation...
This retirement does not include the following:
- Custom Apex REST & SOAP Web Services
- Apex Classes
- Apex Triggers
- Visualforce Pages
Affected managed package versions that are pointing to metadata components less than or equal to version 30.0.
Soapbox Mailer is an on-platform app (which is different than Soapbox Engage, which is an off-platform app). With that caveat, Soapbox Mailer is not impacted by this deprecation or retirement.