The orgtbl-arggregate package (on Melpa and GitHub) does that.
Horizontal lines are converted to empty columns.
Transposing again recreates the original horizontal lines.

Beware, as opposed to org-table-transpose-table-at-point, it does not modify the source table, it creates a new transposed table.

Type C-c C-x x in an empty space in the buffer
Then answer "transpose", and follow the wizard.

It is a bonus, undocumented feature of the package.

Le 10/05/2022 à 08:49, Uwe Brauer a écrit :

Hi 

From the docstring of 
org-table-transpose-table-at-point is
Transpose Org table at point and eliminate hlines.

Does anybody know about a, maybe, 3rd party packages that transpose the
table but leaves the horizontal lines intact?

Regards

Uwe Brauer