It would be utterly insane for Labour to support Johnson's deal.
Electorally:
1. 75% of Labour voters in 2017 voted remian in 2016
2. Johnson getting a deal through reunites the insane and centrist wings of the Tory party behind him and neuters the Brexit Party threat, giving him at least 40% in the next GE.
3. Labour voting for Johnson's deal means driving Labour voters who've switch to Lib Dem further into their camp, possibly permanently (definitely while Corbyn is leader), and maybe forces more enthusiastic remainers to the Lib Dems at least temporarily.
Politically:
1. Voting for Johnson's deal means giving an extreme right wing government control of deciding what consumer and workers rights will be retained when EU law no longer applies in the UK.
2. It gives an extreme right wing government control of striking trade deals first with the EU, then with hostile foreign nations, where things environmental protections and food standards will be on the line.
3. It gives an extreme right wing government the opportunity to basically sell the NHS to the US in exchange for Trump's 'great trade deal'.