https://arxiv.org/ftp/arxiv/papers/1706/1706.07030.pdf ) gives an

Now published: Toft et al. 2017 Nature 546, 510.

In article ,
Martin Brown writes:
The tricky questions are "why did it stop making new stars?"

This is a tricky question in general. Many processes for stopping
star formation have been suggested, but their relative importance is
unknown. Probably all of them, and maybe others not yet suggested,
play a role.

In the specific case of MACS J2129.4-0741, the authors suggest that
gas falling into the center of the dark matter halo where the galaxy
resides is shock-heated and therefore too hot to form stars. I am
not convinced this explanation has to be right, but it's plausible.
An active galactic nucleus, which the spectra show is present (but
probably fairly weak at the current epoch), may also be a factor.

or put another way "where did all the dust and gas go?".

Dust is still there. Average visual extinction is about 0.6 mag, and
from reading the image in Fig 3, peak seems to be about 0.9 mag.
Those values are luminosity-weighted, meaning they are probably
underestimates of mass-weighted values. (No knock on the authors;
this is a universal problem with such estimates.)

Swept clean by a galactic collision perhaps

No evidence for a collision, and it's hard to understand how a disk
(which is what this galaxy is) could have survived one.

or related to its unusually high spin?

Not unusually high; the rotation is normal for the galaxy mass. I
don't see how spin would eject dust or gas in any case.

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