According to several French newspapers, e.g. Liberation, the explanation is that the 2020 date was a typo.
Plusieurs affiches sur le pass sanitaire indiquent une date au 20 janvier 2020, avant les premières mesures sanitaires en France. Elle est tout simplement due à une faute de frappe lors de la création du document, corrigée depuis.
Several posters about the health pass indicate a date of 20 January 2020, before the first health measures in France. This is simply due to a typing error when the document was created, which has since been corrected.
In slightly more detail, it seems the all posters were made on top of a re-used InDesign template, which wasn't updated.
Le gabarit (sur le logiciel de graphisme Adobe InDesign, c’est un fond de document qui permet de conserver les mêmes éléments comme les en-têtes et pieds de pages pour différents visuels) utilisé pour les affiches sur les mesures barrières était erroné, à cause d’une «faute de frappe». «Le copyright sur l’affiche des gestes barrières devait être “20 janvier 2021” et non “20 janvier 2020”», explique le cabinet du Premier ministre, qui plaide «l’erreur humaine».
The template (in the Adobe InDesign graphic design software, it is a document background that allows to keep the same elements such as headers and footers for different visuals) used for the 'barrier measures posters' was wrong, because of a "typing error". "The copyright on the 'barrier measures' poster should have been "20 January 2021" and not "20 January 2020"," explained the Prime Minister's office, pleading "human error".
As additional proof that this was a copy-paste error, Liberation says that in the original PDF posted (now only available on the Wayback Machine), there is an additional error, namely that the title field in the PDF properties was not updated to mention the "health pass" but had the old title of "barrier gestures" that were featured in another poster.
C’est sur ce même gabarit erroné qu’a été conçue la fameuse affiche sur le pass sanitaire. La preuve ? Dans le fichier PDF original du pass sanitaire, maintenant supprimé du site du gouvernement mais consultable dans les archives du site WayBackMachine, le titre du document mentionnait toujours «gestes barrières français» (au lieu du pass sanitaire).
The same erroneous template was used to design the famous poster on the health pass. The proof? In the original PDF file for the health pass, now removed from the government website but available in the WayBackMachine archives, the title of the document still reads "French barrier gestures" (instead of the health pass).
My screenshot (with evince) of the properties of the fumbled version via wayback machine linked by Liberation:
The title field matches that of earlier posters properly on "barrier gestures", from March 2020, such as this one.
Also note that the catalogue entry for the first "barrier gestures" poster has nearly the same verbiage (highlighted below) as in the subject field of the infamous poster on the health pass: "les gestes à adopter pour se protéger du coronavirus" in the subject field of the health-pass poster is out of place. The catalogue entry for the gestures poster has the verbiage
"les gestes à adopter pour se protéger et protéger les autres du coronavirus" (there's a slight addition there to "protect others").
There are newer versions of this "gestures" poster in which that verbiage can be found in the subject fields inside the PDF as well.
The government website doesn't carry this particular version of the "gestures" anymore, but one (dated 14 October 2020) can be found carrying even the W-0333-001-2003 reference id, which was probably a duplication error as well; I think the reference ids are/were also supposed to be unique to each poster.