I've read through these links. They are not conclusive, some sag on one front side, some on one rear side, some on the whole side, others are level. This is not inherent to the car, otherwise there wouldn't be any level ones.
Honestly, I don't think it is normal. With the Vectra I've owned before sagging was "normal", too. But there was a reason to it, it was known the springs Opel used in that car were incredible low quality and very prone to breaking. And it could be fixed for good by simply getting cheap aftermarket springs installed, which were better quality wise.
At best it shows it is not a problem rarely encountered, and some dealerships don't like to deal with it, as they use different excuses. Personally I'm not buying into this. A sagging car means wrong weight distribution which can lead to bad handling. Also, if a broken spring is the cause it is outright dangerous. The spring can turn into its seat, especially in the front and push out the broken bit which can slice the tyre open, leading to a sudden decompression I certainly don't want to encounter while driving on the motorway. There are several incidents recorded for the Vectra, which in turn leads to a very thorough check of the springs at TÜV/Dekra (our kind of MOT). Therefore I would always look into the cause of sagging, as most likely it is a sign of some failure or at least abnormal wear of the suspension.
Also for the record, I got curious and checked mine, although not a CTR and less then 1k miles. I put the car on a level surface , checked with the spirit level, and then measured with the sprit level from the top of tyres to the wheel arch. The measurements are almost perfectly level on front and rear.