gnumeric no longer offers to save as csv/tsv etc (text), despite listing such files when choosing "All files" as save type

.tsv .csv etc files are not full spreadsheet formats, so you can not 'save as' such a file. This does not stop you from importing from or exporting to those kind of file types. Just use the data->importdata or data->exportdata menu items.

Note that you will need to use the configurable text exporter to write a .tsv file.

Thanks. I think these options simply moved (?).
I'm rejecting my bug...

The documentation still says to do Save As and choose "Text (configurable)" as the format. It seems like Data : Export just opens the Save As dialog with this text format available.

I ran into this same issue. It's a bug in the feature's discoverability, and it could be solved by adding a notice below the "File type:" pop-up menu stating that other file formats are available in Data > Export.

Status changed to 'Confirmed' because the bug affects multiple users.

Is there any reason why the save as dialog doesn't support exporting as well?

To myself (and I would guess to many if not most other users), "Save-as" is the go to location for making a file. Splitting export vs save-as seems like forcing the user interface to conform to the internals of the program instead of having the user interface reflect how the program will be used.

Please consider changing the save-as dialog to allow exporting from there.

If not, at least include a note explaining how to export in the save-as dialog.

I note that in addition to the generally counterintuitive location of the export function (under data rather than file, where this option is almost universally found), if a user opens a tab delimited or comma separated file, and attempts to save it, it will default to saving in a spreadsheet format, but will retain the .txt extension, which clearly implies that the format remains unchanged.
Considering the large number of users working in these formats, it seems the obvious answer is to permit saving in multiple formats from the save-as dialog, as expected by the majority of users, but a notice indicating how this can be accomplished should absolutely be included.

It would greatly improve usability to have the csv option visible from the File -> Save As menu.

This is a discovery bug:
too hard to discover how to export TSV/CSV.

File->Export is a more normal location.