All indexes on a pg table need updating if any value in the row is modified, that's the write amplification. The indexes are needed for joins, they're not so important for foreign keys.
Certainly checking foreign key constraints works better with indexes, but those are usually the other way around - verifying row exists with primary key, ie only one index. Updates to primary key would benefit from index on foreign key columns, but that's much rarer.
Certainly checking foreign key constraints works better with indexes, but those are usually the other way around - verifying row exists with primary key, ie only one index. Updates to primary key would benefit from index on foreign key columns, but that's much rarer.