BIP 21 — URI Scheme

BIP 21 (URI Scheme) defined the 'bitcoin:' URI format for payment requests — an address plus optional amount, label, and message parameters — which underlies most wallet QR codes. Despite near-universal deployment in practice, its repository status under the current process is Closed; BIP 321 (URI Scheme, status Complete) is the updated successor specification.

Debate relevance: Appears in UX and interoperability discussions, and in payment-standardization arguments (e.g., how to encode multiple address types or Lightning fallbacks in one QR code). Common misstatements: "BIP 21 is dead" from reading its Closed status — the format remains in wide use; Closed here reflects supersession by BIP 321, not abandonment of the scheme.