I'm inclined to think that #193 was a mistake and should be undone, rather than going further down that path.

Are .tofu and .tf files the same or not?

• if yes, then folk can either use the .tf extension all along, or else set up their own filetype detection
• if no, then it is a mistake to treat them as if they are the same

What I don't want is to slip gradually into accidental support for a non-terraform filetype

Are .tofu and .tf files the same or not?

They are the same. OpenTofu prefers .tofu over .tf files if they have the same base name, but otherwise OpenTofu reads both. Terraform ignores .tofu files obviously.
Ref: https://opentofu.org/docs/language/files/

ps: I'm not a user of OpenTofu though, just wanted to add clarity FWIW.