Per-account state

Restrictions and exemptions are per-account, so knowing what a token can do is not the same as knowing what it will do to one particular address. This answers that in a single call.

AccountState memory state = IERC20AccountState(token).accountState(account);

// state.frozen        - barred from transferring
// state.feeExempt     - transfers touching this account are not charged
// state.configuredAt  - when any of the above last changed, 0 if never

Fields belonging to modules that are not installed read as false rather than reverting, so the call is safe on any token in the framework regardless of what it declares. A token with no restriction module reports every account as unfrozen, which is true.

configuredAt is the one to watch

It is a timestamp, set whenever an authority changes anything account-specific, and zero if nothing ever has. It is the cheapest way to detect that a counterparty's status moved since you last looked, without diffing every field or subscribing to events.

A frozen account is not a static fact. If you hold a position against a counterparty, treat frozen as a signal that its balance can be burned out from under you — see pause and blocklist.