Dictionary: "End-of-life" (EOL) is a term used with respect to a product supplied to customers, indicating that the product is in the end of its useful life (from the vendor's point of view), and a vendor stops marketing, selling, or rework sustaining it.
What Zac meant by the term, I think, is that Microsoft's plans for future Windows 10 Mobile development are essentially zero. In other words, the UI and feature set of the core OS will stay as-is until the actual End Of Life, which will be when the final OS branch reaches its 28 month anniversary. And that's not going to be until some time in 2020, well over two years from now.
But though the current OS feature set won't change, there are continuing enhancements to all the core UWP applications, thanks to commonality with Windows 10 on the desktop/laptop/XBox, etc. Then there are all the under the hood security fixes and optimisations to Edge (for example).
(Plus new and updated third party UWP applications.)
Looking at that EOL definition again, the last phrase definitely doesn't apply - the marketing and selling (of Lumias) may well have stopped from Microsoft's end, but there's very definitely work going on to sustain and support Windows 10 Mobile.
So maybe Zac meant EOD (End Of Development) or EOM (End Of Manufacturing)? Anyway, all of this got me virtually sketching and charting:
The most obvious thing to note is that the various Windows 10 Mobile branches are only mid-cycle, at most, in terms of support - all the phones which were pushed the W10 Fall Creators Update are in it for the long haul, full supported with patches and fixes as shown, until the end of 2019 (at least).
Of course, there's more to owning a phone than OS support - what about the hardware itself, should it break or need replacing? The Lumia 950 range, as shown above, is long out of production now and hard to repair or replace officially, which is why that Alcatel IDOL 4 Pro is such a significant phone and worthy of your consideration. It's still actively sold, is (in theory) fully repairable and replaceable and is a true flagship (at least in this company), imaging excepted, since it can't live up to the Lumias that went before.
But yes, the range of available hardware is limited in the extreme. And it's about now that peoples' favourite Lumia 930s and 1520s are starting to fail - they may be finishing life on the Anniversary Update, but even that gives them until the end of next year - assuming the hardware keeps on chugging away.
Are you convinced then? I contend that Windows 10 Mobile is very definitely not even close to 'End Of Life'. I know what Zac meant though - will you accept 'End Of (active) Development'?