Hi @stokerg and thanks for your answer.
I saw what you linked, but it doesn't help me at all. First, I can't accept the answer "The best solution is to reduce the DPI/PPI export setting. No one is going to view an image of that size". To give you my example, I work on long image strips, and we read them on specific readers by scrolling. Its frequent to have a big height, and I absolutely don't want to reduce the quality of the picture!
There may be a GDI limitation (but not for the exporting, unless you use the GDI to export in PNG, but you may easly use something else), but I don't see why Affinity would be unable to bypass this limitation. As others have said, PNG filetype doesn't have such a limitation.
Lastly, the link speaking about a limitation isn't sourced and is 15 years old. Furthermore, it doesn't mention a limit in exporting, but simply a limit for viewing.