So I'm having another go - see below. If you don't want Labour to back a Tory Brexit and be disqualified from complaining about its damaging effects, you should write to Sir Keir and your Labour MP if you have one.
Dear Sir Keir, hope you had a good Christmas. I am disappointed not to have received a reply to my email of 6 December (see below) particularly as I keep reading that you are going to instruct Labour MPs to support Boris Johnson's dreadful Brexit 'deal'.
Every day, new details emerge of how it will damage people's lives, but if Labour votes for it, you will be disqualified from criticising its effects. Imagine the scene :
'He used to be Captain Hindsight, Mr Speaker. Now he's been demoted to Sgt Turncoat! Just a few days ago, he and the party opposite voted for our historic agreement with the EU. Now they're against it! I know he's a lawyer, who changes his brief as often as he changes his briefs, Mr Speaker, but he's no leader. Britain needs leadership, and the party opposite have shown once again that they're shallow, unprincipled opportunists, who have nothing to offer our country.'
In 2018, you promised me Labour would vote against any 'deal' that did not satisfy all its six tests. This one comes nowhere near. You should keep you promise and urge Labour MPs to vote 'no' to Johnson's deal.