I don't think that quite applies. Technical debt comes from making choices that help you in the short term but cost you more in the long run (i.e. you're paying interest). Needing to move off a deprecated tech stack wouldn't really qualify unless there was a longer-term option available at the time you originally decided on your tech stack.

Would it really be technical debt when we talk about transition from py2->py3? Technical debt is about the consequences of bad design. But py3 is more the consequence of growth and gain of experience. And for a company it is an external debt.

I would call it more something like upgrade debt or time debt, as it's a problem that appeared because an external situation changed over time. Similar problem in that regard are operating systems which change and let old software behind, or in case of linux old packages not working anymore, repos disappearing, etc.

py2 to py3 would be handling the technical depreciation of value.

This is different. Tech debt is a factor of letting things get slowly worse. Those things can be improved if you just work on your tech debt.

What I'm talking about can't be improved or fixed, or higher-ups refuse to improve or fix it. Instead, it has to be completely replaced. It's like a Replicant with a four-year lifespan. You literally know it's going to die in four years, and nobody orders a new one until the old one is practically dead, and everyone has to rush to replace it.

Sounds like Egyptian emperors and pyramids

there almost is, an irony mark or percontation point "⸮" https://en.wikipedia.org/wiki/Irony_punctuation