According to James Forsyth:
Khalid Mahmood has resigned from the Labour front bench saying that 'A London-based bourgeoisie, with the support of brigades of woke social media warriors, has effectively captured the party' source
This comes after the historic election loss for Labour in local elections, where the party bled even more in working-class areas than it did in 2019.
Haven't looked deeper, but "quitting the labour front bench" doesn't mean leaving labour, it means becoming "a back bencher", still presumably doing the work of an MP but no longer being part of the "front rank" or the shadow cabinet etc, or something along those lines. I forget the specifics off the top of my head.
Back benchers retain party affiliation, but are less directly under the control of whoever is nominally in charge of the party at the time, and are much more free to vote against party line, etc.