Source Code shows that it is only used for Tomcat 6. This is tied with Spring LTW (LoadTimeWeaving) functionality.
From Spring Documentation, it is not needed for Tomcat 8.0 and higher.
Do not define TomcatInstrumentableClassLoader anymore on Tomcat 8.0 and higher. Instead, let Spring automatically use Tomcat’s new native InstrumentableClassLoader facility through the TomcatLoadTimeWeaver strategy.