We don't usually do this

FYI we tend to always create an abstract parent class when we expect the user to have to implement an interface of ours. That ensures that - in certain conditions - it gets easier for us to add new methods to the SPI without breaking backwards compatibility.

See for example org.hibernate.search.backend.impl.jms.AbstractJMSHibernateSearchController, org.hibernate.engine.spi.SessionDelegatorBaseImpl, HHH-8901, ...