>if the software contains a signature meant to prevent users from enjoying those freedoms and that control, in the intended (possibly single) use scenario, would it be reasonable to say that the software is free?
The software is rendered proprietary by digital handcuffs in that case, which is something else to Tivotization.
What Tivo originally did was to provide the source code and installation information for all the free software - but make all the proprietary software stop running (you would have a DVR that could run VLC, but you couldn't run any of Tivo's proprietary software for the purposes of inspecting it/the RAM contents to see what spying it did and for its replacement) - which the GPLv2 does nothing about, as the GPLv2 doesn't contain any requirement to agree to not sabotage the execution of the system's aggregated software; https://sfconservancy.org/blog/2021/jul/23/tivoization-and-the-gpl-right-to-install/
Tivo later decided to intentionally infringe copyright by infringing the GPLv2 with future Tivo models, by refusing to provide the complete corresponding source code and installation information (the software was handcuffed - you needed a signing key for it to execute) and as a result, Tivotization is commonly confused with Tivo's GPLv2 infringement.
>I don't think the case of SOF is one of digital handcuffs.
It is a case of digital handcuffs - there is source code that is claimed to correspond to the binaries and it can be compiled into a binary - but you can't run it on most of the relevant sound cards, as you need a private key that Intel refuses to provide (the binary can be made to execute on some SBC's and certain chromebooks, as Intel has provided the private key, but of course other parts of those are handcuffed).
It isn't a case of Tivotization, as with the necessary private key, the user could install a free DSP program and all the software on the computer would continue to execute without any of the proprietary software ceasing to function.
