We have just one term

For goodness sake people, it’s Ts and Cs not T and Cs.

Nor is it T and C’s or, T’s and C’s!

Feel free to replace ‘and’ with an ampersand (you know, one of these things: &) if you'd like to try and push back a little on this case!

Anyway, clearly there’s no ownership of these words, there are just many of them. So, an apostrophe isn’t required; but, to show many, an ‘s’ is required!

Written as it get spoken that would then be written as, Tees and Cees. So as much as people no doubt say, ‘Even though there’s only one T in T&Cs, everyone knows to say Tees and Cees… it’s just a thing…’ it should (surely?!) be written as: Ts and Cs; or, Ts&Cs!

Unless of course all these companies have only one term, but many conditions that their customers must follow.

But, it’s more likely most do it because other companies do it like that, so it must be mostly official/expected/correct and no one likes being (too) different (and/or correcting false perceptions) in the school playground, apologies, company meeting room.