From my experience of many trips in Europe, Africa, North America and Asia, the issue of diesel quality is overrated and often overdramatized. On the subject of low-sulfur diesel, you can take a look at this map, for example:
Global Sulphur Levels
Nevertheless, it should not be forgotten that poor quality or even contaminated fuel can lead to expensive complications. In critical areas I would change the diesel filter more often (carry a replacement), keep an eye on the water separator and possibly use a pre-filter (e.g. the well-known MrFunnel funnel filter) or install a 2nd fuel filter. What has also proven successful for me is (assuming appropriate range) to refuel only at gas stations that look trustworthy. Over time, you get a certain feeling for it.
By the way, I've only had problems with diesel once on my many trips, and that was in North America - I've never had so much water in the separator.
If you are going to cold areas, it can make sense to carry an additive to make the diesel resistant against freezing temperatures, not all countries in the world have winter diesel. I also recommend, in certain overlanding regions, to inform yourself on the subject of additives.