IMHO the fact that it's hard to get TLS 1.2 (or indeed any form of "pure" cryptography) working at all on an obscure platform says a lot about the state of commonly available crypto libraries --- as those who are against regulating it like to say, it's "just math"; and math that any computer should theoretically be able to do. OpenSSL is one of the more portable ones, yet I'm not sure how it would be able to handle the 64K segmented model. It would need at least 32KB (maximum TLS record size, multiplied by both directions) and a little bit more per TLS connection. That said, a 2048-bit RSA key is "only" 256 bytes, and ECDH ones are smaller, so I think a TLS 1.2 (or even 1.3) implementation on the platform is definitely in the area of "feasible, but not trivial". A lot of IoT stuff has similar constraints (minus the segmented addressing).

If someone has got a working TLS 1.2 on such an old OS, I'll like to know too. Because I certainly haven't saw any example.

I think the work to get TLS 1.2 working will certainly dwarf the work on this Slack app.

Yes I was curious about that too. Cheater. :)

Seems to me you'd pretty much have to write a simple TLS 1.2 or 1.3 client by hand, or port an existing simple implementation.

But yeah, makes sense as it would be way too much work.