Deselection imo is for those who choose on key HoC votes to oppose manifesto policy and disobey 3-line whips to do so
and certainly for those who vote the "wrong" way on No Confidence
An MP is entitled to use their own judgement how to vote and campaign,
but there is a limit to how far any party can tolerate deviation from key official party policy in actual voting, especially votes that could lead to a GE.
If the Tory conference and new PM decide on No Deal and the PM makes key votes on this into 3-line whip,
then any MPs who vote against No Deal, or to bring down the govt in an NC vote, should obviously expect deselection
Similarly, any Labour MP who votes with the govt on NC or No Deal should be deselected