It seems the manifesto was quite artfully written to allow an interpretation where this is not a broken promise:

Page 52 of Labour's manifesto reads like a rhetorical twister – a paragraph of promises seeking to stretch in multiple directions at the same time, trying to keep a breadth of people happy simultaneously, or perhaps not annoy too many people too much.

"Labour will not revoke existing licences and we will partner with business and workers to manage our existing fields for the entirety of their lifespan. Crucially, oil and gas production in the North Sea will be with us for decades to come, and the North Sea will be managed in a way that does not jeopardise jobs," it reads.

"And our offshore workers will lead the world in the industries of the future. We will not issue new licences to explore new fields because they will not take a penny off bills, cannot make us energy secure, and will only accelerate the worsening climate crisis," it adds.

BBC News - Chris Mason: Jackdaw approved but tricky energy trade-offs remain - BBC News https://www.bbc.co.uk/news/articles/cked35dz47y7o