Cross-wiki blocks are not properly implemented. It's currently basically only possible to insert blocks cross-wiki. It's not possible to update or delete those. It's also not possible to create cross-wiki blocks containing restrictions since those are then being stored in the local database (which is something we defenetly need to change). This is also the root cause of T281972: ActorStore::checkDatabaseDomain: InvalidArgumentException: DB connection domain does not match when suppressing via Special:CentralAuth (CVE-2021-36128).

Rough steps of implementing cross-wiki support for blocks.

1. Make Blocks wiki-aware - T274817: Convert DatabaseBlock and AbstractBlock to UserIdentity, and make them cross-wiki aware.
2. Create a BlockRestrictionStoreFactory - T291983: Create a BlockRestrictionStoreFactory in order to make BlockRestrictionStore a proper cross-wiki store
3. Create a DatabaseBlockStoreFactory - T291849: Create a DatabaseBlockStoreFactory in order to make DatabaseBlockStore a proper cross-wiki store
4. Implement cross-wiki support in the BlockUser class.

Things need to be done when cross-wiki blocking is possible.

1. Update CentralAuth to properly perform cross-wiki suppressions.
2. Fix CheckUser autoblocking, see T281972#7078969.