It's package-private and has been unused since the removal in 2.6 of setApplicationContextClass:
https://github.com/spring-projects/spring-boot/blob/f185b0767a71c2c326769bafea5966b677d6be00/spring-boot-project/spring-boot/src/main/java/org/springframework/boot/SpringApplication.java#L1238-L1251