Since the Labour Party's fledgling days, there's been a battle between the left and not-so-left over who should run the party and what policies it should follow.
The not-so-left seem convinced the Labour Party should be the sole preserve of them and them alone, and there's been periods of expulsions of people from the left for not fitting the desired narrow political profile that the not-so-left want.
I prefer using the term 'right' and 'left' to describe the differing socialist tendencies inside the Labour party. It's much clearer - at least for the period when right and left were understood in
economic terms (ie up to the mid 1980s). So what you call the "not so left" is better understood as "right".
The "periods of expulsions" you refer to fall into three main periods I suppose (four of you include what Starmer is hoping to do today). The first was between 1921 and 1926, the second between 1945-50, and the third in the mid-1980s. Each time, interestingly, the initiative came from a left-leaning leadership (MacDonald, Attlee, Kinnock). The periods of right-leaning leadership were not accompanied by many expulsions (Clynes, Gaitskell, Callaghan, Blair.)
Were people expelled because, as you claim, they did not fit "the desired narrow political profile" that the leadership wanted? I'd say definitely not. Any political party that can embrace Jimmy Thomas and James Maxton and the ILP-ers (in the 1920s), or Herbert Morrison and Nye Bevan (between 1945-51) or John Golding and the Bennites (in the 1980s) cannot be called "narrow". The idea is bonkers!
Each time the expulsions were about authority. Whose authority should Labour members accept? Should it be the authority of the Labour party or should it be the authority of another party, operating secretly or in direct opposition to Labour at the polls? In the early 1920s those expelled preferred to accept the authority of the new Communist Party of Great Britain (or, in reality, the authority of Moscow). They had to go. Between 1945 and 1950 it was the same problem. Some members, including a handful of sitting Labour MPs, were booted out of the party because they preferred to take their instructions from Stalin and the Politburo. They obviously had to go too. In the 1980s, of course, the members who were expelled largely came from Militant. These expellees claimed Militant was just a newspaper, but it was a political party with its own annual conference and executive committee, policy committee and local branches. They had to go.
There's nothing "narrow" about the Labour party, either then or now. It has always been a broad church. It still is. Plenty of room for everyone who believes in a parliamentary road to socialism.