Is it walking through the grey zones, 'SILT', protection around event sites, where you need a QR code permit matching photo ID you are concerned about? Ignore red zones, they are vehicle bans not pedestrians.
... on the way back I arrive at Austerlitz and it is slap bang on the middle of the restrictive zone.
Gare d'Austerlitz is
not in the grey zones, looking at the maps. Between Gare de Lyon and the river is in the grey zone for the opening ceremony but not after. The embankments are but the maps suggest you can still walk across Pont Charles de Gaulle.
From what I have seen in the news, if you are staying in a genuine hotel in a SILT you can get a QR code but it must be done in advance.
As well as 30907's reference, the Ville de Paris site (in English) with downloadable PDF maps of the zones is:
Find out all about the opening ceremony of the Paris 2024 Olympic Games. Key dates in the organization of the event, security perimeters and impact on transport.
www.paris.fr