DmD 2021 - Hints
The environmental sticker (Crit'air sticky) is only required in the event of an air pollution crisis in large cities such as Lyon, Grenoble, Paris, but also in the Puy-de-Dôme department. In case of an alert, every car or motorcycle, regardless of its country of origin, needs a sticker when entering such a "green zone".
However, an alarm is unlikely to occur during the DmD - no alert has been triggered so far in the Puy-de-Dôme department. On the other hand the costs of 4,18€ for the badge seem to be manageable.
Should your route possibly take you through such a green zone, which was decided at short notice, then you should consider getting a sticker.
If you think you have to get through theses green zones during your trip and that would happen during "pollution crisis" may be you should get one.
A badge can be obtained from the following site: https://www.certificat-air.gouv.fr.
There is the "Green Zones App" https://www.green-zones.eu/en/web-apps-und-datenservice/die-green-zones-app.html for Android, iPhone and iPad, available via the corresponding stores (Google Play, App Store).