Previously, you could indicate you wanted an index on a field, but only on a single field, and you did it in the field definition.
Declaring them in Meta via the Index class is new, as is the multi-field support and direction support. More importantly, you can subclass Index to define other types of indices besides the default (which is btree). For example, Django now ships built-in subclasses to do BRIN and GIN on Postgres.
Previously, you could indicate you wanted an index on a field, but only on a single field, and you did it in the field definition.
Declaring them in Meta via the Index class is new, as is the multi-field support and direction support. More importantly, you can subclass Index to define other types of indices besides the default (which is btree). For example, Django now ships built-in subclasses to do BRIN and GIN on Postgres.