What is the name of the method for cleaning a database from outdated or unnecessary data to improve its efficiency?

The method of cleaning a database from outdated or unnecessary data to improve its efficiency is called "garbage collection" in the context of memory management, but in databases, the terms "cleanup" or "archiving" are more commonly used.

In relational databases, "vacuum" (for example, in PostgreSQL) is often used — a process that removes dead rows and frees up space.

Overall, such a process is called "cleanup", "vacuuming", or "garbage collection" depending on the context and the type of database.