It’s a bit boring, but OTL 2021 may well force this. Certainly an even more overt left purge by Starmer, so a POD earlier this year, could have seen something like this perhaps already begin to be talked about seriously, a la the many months of setup for TIG’s damp squib.
I agree that the Labour left pre-Corbyn would not go for this. They just didn’t ever seem to see the point. Now, though, having seen that they have more support than they thought and were capable of getting within spitting distance of the levers of power, I think there’s a different mood in the air. I can see some attempt at a big split and a new party, perhaps in the aftermath of a leadership challenge, certainly with a POD of Starmer being even less subtle about purging the Corbyn-era membership.
It doesn’t matter that they would have a ceiling of 20% and would likely go the way of the SDP within a generation (perhaps UKIP-folding-back-into-the-Tories, or Canada’s unite the right movement, is the more appropriate comparison because they would likely merge with the party they split from). What matters is whether it could be attempted, and I think this is a rare case of something probably remaining implausible in the past without big PODs, but becoming more possible as time goes by in the present day. I’m reminded of a thread in the other place, in about 2013-2014, that asked about the possibilities of a Socialist Campaign Group MP winning the Labour leadership. It was broadly laughed out of the room.