What happened to writing the “core” of an app that doesn’t rely on UI then simply writing the front ends for each platform you want to support?

What do you mean? I can't speak for Slack but I'm sure some degree of business logic / client side logic separation exists.

By the way, what you just described is the essence of cross-platform development, rather than an argument for building apps natively.

simply writing the front ends for each platform you want to support?

But why would you rewrite the "front-end" for each platform if you have one you could just port over? Who is going to pay for those 2x developers and what would be the ROI on this effort?

That’s just three (if you don’t write for a million desktops on Linux).

Is it really so hard to support just three environments with only the UI being tailored for the OS it’s running on?

In Slack's case I'd wager the answer to be a resounding YES. I don't think you fully grasp the full scope Slack's capabilities, and the amount of work involved to build native clients for not just one or two, but three different platforms - it's definitely not just the "UI".

Honestly, it just feels like poor tooling and a poor excuse.

Quite the opposite - frameworks like Electron let's devs with your skillset build with the stack you already know, and abstracts away quite a bit of the cross-platform complexities, which strangely enough is what you are suggesting but also what you are arguing against