There is a slight difference with your 0% tax analogy. No political party would ever offer that in a manifesto. Both the last two Conservative manifestos promised to implement the outcome of the referendum, and MPs such as Mr Grieve stood on that platform.
By the time of the 2017 election if I remember rightly May was already propagating the notion “no deal is better than a bad deal” (I forget if that actually made the manifesto in as many words).
I can sort of respect the Lib Dem position as to be fair their position on Brexit has always been pretty much the same - although perhaps they avoided too much awkward scrutiny at the time as the referendum coincided with the period when they had their lowest number of MPs. Labour meanwhile have been completely muddled, but generally attempting to make some attempt at saying they would honour the referendum result (though anyone watching their conferences might raise more than an eyebrow at this!). I think it’s very hard for a Conservative MP to actively promote remain, even in a remain seat - their manifesto was pretty clearly pro-leave. Even someone like Kenneth Clarke is on rather dodgy ground IMO.
Obviously John Major isn’t constrained by having stood on the back of a manifesto, just weighed down by all those chips on the shoulders which have no doubt been quietly growing away since 1997. George Osborne and Philip Hammond are giving him a bit of competition on who can grow them fastest though...