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In 2018, a Metrodecker EV prototype from Optare was trialed on the route.
In November 2021, the frequency of the service was reduced from 6-8 minutes to 8-10 minutes.
On 4 February 2023, route 21 was re-routed to serve Nag's Head in Holloway instead of Newington Green, partially replacing the daytime route 271, following a successful consultation in early 2022.