ScanSupport

What happens to a subscription the provider cannot serve from an index.

The distinction between the first two is not how clever the matching is — it is whether the provider already holds the block's contents. An in-memory ledger, a chain-sync follower and a gRPC stream all see every transaction anyway, so a subscription that matches everything costs them nothing beyond the fan-out they already do. A REST provider whose cheap path is per-address endpoints has to fetch the block and then a UTxO set per transaction in it, which is the difference between a handful of requests a day and a spent quota.

The third is a different kind of answer, and it needs to be stated rather than approximated by the second: a provider built entirely out of per-source lookups has no request sequence that would answer "every transaction" at all. Calling that Metered would let a caller consent, via allowUnindexedScan, to something that cannot happen — and the subscription would then be accepted and deliver nothing, forever, with no error. Consenting to an impossibility is worse than being refused.