In the past (2003), cam.misc has concluded that these year markers on
Fair Street are parish boundary markers for Holy Trinity Parish, and
nothing to do with water levels.
https://cam.misc.narkive.com/fa5xv5EV/high-tide-markers-on-fair-street-newmarket-road
Cites an online article from Cambridge News, that the Wayback Machine
doesn't seem to have captured, so I can't assess the credibility of that
claim.
But the current boundary of Holy Trinity Parish does turn that corner,
so it at least doesn't fail that plausibility test.
https://dioceseofely.maps.arcgis.com/apps/instant/portfolio/index.html?appid=2706c8a653514d2eac14728707ce36b6
The quoted image on Wikimedia Commons, with its suggestive title
https://commons.wikimedia.org/wiki/File:Historical-floods-marks-in-Cambridge-keeping-the-memory-of-the-floods-of-1821-1861-1878-and-1984.jpg
comes from this paper preprint from 2015:
https://doi.org/10.5194/hessd-12-6541-2015
The preprint's vague approach to citation doesn't really allow reviewing
the evidence (if any) that these mark water levels, as it asserts.
(As far as I can tell, this paper was never published. The reviewers of
the preprint were not kind about its rigour in general, and at least one
recommended rejection.)
It should not be hard to determine whether or not there was a Cambridge
flood in 1984 of such magnitude as to inundate houses on Maids Causeway.
(Long before my time, though.)