In other words, remove static access to them.
Scope the BasicType factory/registry to SessionFactory/Metamodel as well. That contract itself is being refactored for 6.0 as well. Prior to 6.0 this is org.hibernate.type.BasicTypeRegistry. In 6.0 this is being refactored into org.hibernate.type.spi.basic.BasicTypeFactory. So depending on where we agree to apply this change will determine the exposed type.