I don't think they do, actually. Longevity sounds good, but in reality anything that's old probably has critical security holes and so you shouldn't use it anyway.

A warning is sufficient. Old tech should continue to work, for preservation and archival reasons.

I've long ago realized that archival needs to be a separate task left to archivists and archive systems. If you take it into account when designing a live system it's liable to seriously compromise your system design.

Say you're making a chat app - you wouldn't incorporate a delete feature, and you might be tempted to use some kind of blockchain to prove all messages were delivered without gaps. But if you ignore archival needs you design something similar to IRC which is much simpler.

That depends on your user base. If your group of target users includes professional, corporate, or governmental use, then you absolutely need to build in signing and archival for legal reasons. If your users include people who may only connect occasionally or with flaky connections, then you need a robust way to ensure that messages are held for delivery and that all messages are delivered in order. Basic chat (without delivery guarantees or archival) is already solved by IRC, long ago.

just archiving binary artifacts and source packages is enough

Reproducibility adds nothing here

It sure does: no need to keep the binaries around if they are reproducible.

You probably don’t know how industrial computing works.

Toolchains absolutely need to be maintained with some degree of longevity.

The whole world doesn’t march to your consumer-user beat. Sometimes it functions at industrial-user tempo’s, too ..

You’re not thinking like an industrial user but rather as a consumer. Maybe you should extend your scope a little bit.

Industry is learning - often the hard way that out of date software is only acceptable if the device is not connected to a network at all. Even government labs with a separate top secret network that isn't supposed to be connected to anything else get hacked from the internet.

Not that you are wrong, industry keeps thinking they can make themselves immune and so long term reproducibility is useful, but I submit they are wrong.

Disclaimer: I work in the safety-critical/industrial sector of software.

Literally none of your statements are applicable to that realm, sorry.

Rail operators have long since been operating their air-gapped infrastructure with 99.999% safety results, literally not adhering to any of the policies you claim are endemic to the industry.