Does @Document/@Field mapping enforce a schema the way JPA's @Entity does for a relational table?

No — MongoDB itself enforces no schema at the database level, so @Document/@Field mapping is purely a Java-side convention for how Spring converts your object to and from BSON. MongoDB will happily store a document that doesn't match the Java class's shape at all, which is why schema drift in a Spring Data MongoDB app tends to surface as a deserialization error rather than a database-level rejection.