we (mis)use this field in several projects, not for a fax number, but for other kind of data

I think it is not a good idea to delete core survey fields. Fax is surely not used much any more. But for backwards compatibility, hiding it at the interface would be a starting point. Fully removing the field can break many things and should only be done at major version updates e.g. v 6.x.

I agree, this should have been done in a major version - I am really sorry for that.
However, this change has been done for months now and nobody else complained about it, so we won't roll it back.

I can see the commit https://github.com/LimeSurvey/LimeSurvey/commits/master/installer/create-database.php
but this important change has not been mentioned in the release notes https://github.com/LimeSurvey/LimeSurvey/blob/master/docs/release_notes.txt

I dont notice every commit, but I read the release notes on a regular basis

I hope this only affects LS 5.x and not LS 3.x as well?