Monotype Columbus by Patricia Saunders seems to be a well-known and beautiful font (identifont.com page), but I can't find it for sale anywhere anymore. All the main shops include residuals, e.g. Linotype has the navbar, and fonts.com still has a whole directory, but "0 styles available".

I know without doubt that it was available for sale last year when my colleague bought it at fonts.com.

I have been trying to contact Monotype but I cannot get an answer. Only "that font is not available, look at all our other fonts". Does anyone know what really happened, and especially, where we can still buy this font?

According to this article Patricia Saunders died in 2019.

https://research.reading.ac.uk/women-in-type/patricia-saunders-1933-2019/

Maybe as some sort of tribute to her monotype pulled the typeface from sales?

What is also possible is that over the past few months Monotype has been updating significant portions of their library, including Linotype and ITC libraries. In most cases they are not updating the font itself, but rather updating the copyright and description info. A great number of the fonts were produced with Adobe, and so there has been a pretty significant effort to "de-Adobe" the older fonts. This means one of two consequences for the Columbus MT family: either the "updated" family will soon be released, or perhaps Monotype and Adobe has unresolved copyright issues with that specific family.