When I open bitcoin-qt, these are the default widths (note: I'm using the Interlingua language; I haven't tested with English as UI language):
Widths when starting bitcoin-qt:
[
]1
My changed widths, which it doesn't remember after quit:
[
]2
Hi @Geremia, I think the column widths are now saved and restored in current master since the merge of https://github.com/bitcoin-core/gui/pull/205. If you like, perhaps try building and testing on master branch and letting us know.
Also, "Requested payments history" and "Peers" column widths aren't remembered.
https://github.com/bitcoin-core/gui/pull/205#issuecomment-787095485