TLDR: You can use another filing method if you inform the administration that you are unable to file online. An exemption related to the absence of mobile service is also provided for in the legal extract supplied, but only until December 31, 2024; its current application is therefore not confirmed by these sources.
General rule
The return provided for in Article 170 of the French General Tax Code and its appendices must be filed electronically if your main residence has internet access.
Inability to file online
You can use the other means provided for by law if you inform the administration that you are unable to file your return electronically.
This exception concerns the online filing method. It does not constitute a general exemption from filing your income tax return. The sources provided do not specify what the other filing methods are in practice.
Absence of mobile service
Individuals who reside in an area where no mobile service is available are, according to the extracts provided, exempt from filing their income tax returns electronically until December 31, 2024.
As this date has passed, you cannot consider this exemption to be currently applicable solely on the basis of the sources provided.
Scope of the documented exceptions
The sources provided document these two situations in the context of the obligation to file income tax returns electronically. They do not make it possible to confirm any other exceptions to this obligation.