Nuendo 11 update to Nuendo 12 is on sale for 99€ but I already have Nuendo 12.
Is it still possible to buy this Nuendo 11 update and use it for Nuendo 13 when its released?
I know people used to do that on old licencing but I am not sure its still possible on this new one.
You’d have to own a lower version than 11, to update it. See if you can find a used copy online…
Almost certainly the answer is “no”. This sort of thing was not possible on the old licensing system either, contrary to your assertion - eLicenser updates were strict on which versions were qualifying licences for an update or upgrade.
A further complication is that Nuendo 12 is the first version of Nuendo on Steinberg Licensing. An update intended to upgrade Nuendo 11 or earlier will be looking for the old licence on eLicenser, not Steinberg Licensing.