Lightning mainnet launch

The Lightning Network reached mainnet-usable status in 2018: Lightning Labs released lnd 0.4-beta on March 15, 2018, the first Lightning implementation declared beta (feature-complete and safe enough) for real funds on Bitcoin mainnet. The concept predates this — the Poon-Dryja Lightning paper circulated in 2015, and adventurous users ran pre-beta mainnet channels in late 2017/early 2018 — so "Lightning launched in 2018" is correct for the mainnet-beta milestone, with the paper dated 2015.

Nuance: Lightning has no single launch: it is an open network with multiple interoperable implementations (lnd, Core Lightning, Eclair, LDK) speaking the BOLT specifications. The 2018 date marks the first implementation's mainnet beta, the conventional anchor.

Common misstatements:

• "Lightning was part of Bitcoin's original design." — The whitepaper predates it by seven years; Lightning's design was published in 2015 and required SegWit's malleability fix to be practical.
• "Lightning launched with SegWit in 2017." — SegWit (August 2017) was a prerequisite; the first mainnet beta came in March 2018.