I think I measure tech debt by how easy or hard a piece of code is to change, in response to changing requirements. So yes, I’d agree that if code is never going to be changed again, then the concept of tech debt is less-applicable.

That said, predicting which code will or won’t be changed in the future is a difficult prognostication. It seems safer to assume it will be changed on a long enough time span. Then if it’s not, you can be pleasantly surprised.

I tell people, code architecture is designing in flexibility you will never use. It's a pretty safe bet you won't guess right.