There’s a third one, and it’s non-tech companies or companies for whom software is not a core product. They only make in-house tooling, ERP extensions, etc. Similar to your Twitter example, once the ERP or whatever is “done” there’s not much more work to do outside of updating for tax & legal changes, or if the business launches new products, opens a new location, etc.

I’ve built several of such tools where I work. We don’t even have a dev team, it’s just IT Ops, and all of what I’ve built is effectively “done” software unless the business changes.

I suspect there’s a lot of that out there in the world.