MongoHealthIndicator and MongoReactiveHealthIndicator are both in spring-boot-data-mongodb because they use MongoTemplate and ReactiveMongoTemplate respectively. However, their use of the templates is really light and I think could be reworked to use the MongoDB Java Driver directly. Both could then move into spring-boot-mongodb.
If removing the Data dependency proves harder than anticipated, we should review the property name. After #47050 it will be management.health.mongodb.enabled. Should it be management.health.data.mongodb.enabled instead?