Here are the tweets anyway - I realise that building stories out of tweets is risky, given recent headlines, but in this case it's merely confirming what most of us already knew or hoped:
The Windows Insider preview program for Mobile will continue.— Jason Howard (@NorthFaceHiker) October 26, 2017
Yes we plan to keep releasing new builds for Mobile!— Brandon LeBlanc (@brandonleblanc) October 12, 2017
A little recap. The Windows 10 Fall Creators Update (FCU, a.k.a. Redstone 3, the '1709' branch of the OS) is rolling out, for desktop and for mobile, with an official 'supported devices' list that includes the HP Elite x3, Microsoft Lumia 550/650/950/950 XL, and Alcatel IDOL 4S/4 Pro/Fierce XL, plus some other carrier phones you've probably never heard of. And we know from this story that Lumias (in particular) are supported for 'feature updates' for 26 months from first availability.
Now, although it's true that 'Redstone 3' for Mobile has very much less new on the surface than on the Desktop, since it's missing all the Mixed Reality and 3D stuff, for example, plus lots that's specific to the x86 Explorer shell, it does still have quite a bit that's common under the hood, not least the latest security and bug fixes. Still, Microsoft chose to name the last few Fast ring builds along the lines of 'feature2_rs3', so it's clearly aligning modules in some way.
But what of 'Redstone 4', probable branch number '1803' (as in 2018, March)? We're seeing the x86 Explorer shell for the desktop marching ahead again, with tweaks all of its own - and we're hoping that much of this comes to a future unifying Windows 10 branch, probably on Snapdragon 835/845 chipsets in 2018 - but what of 'Mobile'? This weekend's tweets have confirmed that the Insider programme will continue for Mobile, implying that there will be test builds after 'feature2_rs3'.
In other words, 'feature2_rs4' or similar, again bringing in core fixes and security updates in parallel to the desktop, but without really adding anything to the existing user interface on Windows 10 Mobile phones. From the links above, we know that the Lumia 950/XL is guaranteed feature updates for 26 months from first availability, which takes us to December 2017, and the Lumia 550 and 650 weren't that far behind, again completing their 26 month window before Redstone 4 is finished. Thus, although these smartphones all got pushed FCU recently, it's very unlikely that the 1803 branch will get pushed in the same way. From now on, beyond FCU, it's small fixes and security updates only for these phones. (You could argue that this is all the 'feature2_rs3' builds are, mind you!)
But the Insiders programme always stays open to older phones for longer - typically up to six months after official feature updates stop, and often up to a year. Which is why the likes of the Lumia 735, 830, 930 and 1520 officially stopped in terms of feature updates on 'Anniversary Update' (1607, i.e. July 2016, codenamed Redstone) but they were 'provisioned' for the Insider programme for another six or so months, so that users of these phones could help beta-test the 'Creators Update' (1703, i.e. March 2017, codenamed 'Redstone 2').
'Provisioned'? We hear that word a lot in the online update world, it just means that there's a master database on a manufacturer's server. And when a phone comes a calling over-the-air, asking for new updates, the phone's model number and variant and Insider programme status on a specific OS branch are all checked and, if all is well, an update is made available. If something's wrong, such as the phone being too old or not allowed explicitly by Microsoft on the Insider programme for that branch, then sorry, no updates for you, etc.
Thus we've heard that the Lumia 640 and 640 XL, in addition to being too old for the FCU (hey they scraped into CU by the skin of their teeth in terms of device age), are also now too old for the Insider programme at all. And, famously, the likes of the Lumia 830 and 930 were allowed into the Release Preview Insiders ring for CU but no further (i.e. Fast and Slow rings, leading to FCU). You get the idea.
So, although the classic Lumia 950 and 950 XL, for example (plus the 550 and 650), won't be getting the 'rs4' branch pushed to production phones, it's now a dead cert that they'll be provisioned for the Insider programme for at least another six months. And because we're not expecting a ground-breaking UI or shell shift for Windows 10 Mobile at this point, there's no real reason why they shouldn't be allowed on all three Insider rings (Release Preview, Slow, and Fast), with the latter being the builds with more bugs fixed at the (now very slight) risk of instability. So if you play along at home then you can bump these phones up to 'feature2_rs4' (or whatever Microsoft calls it) and, at some point in Spring 2018, leave the Insider programme and be on the '1803' branch and thus then supported with bug fixes and security updates until Spring 2020.
All of which is a slightly long winded (though precise!) way of saying that your late-2015/2016 Windows 10 Mobile phones (Lumia 550, 650, 950) can all be made to run whatever's next in Windows 10 Mobile.* The likes of the Elite x3, Alcatel IDOL 4S and IDOL 4 Pro are newer again, of course, and will pick all this up in their production status.
* What's next? Ah yes. Nothing very exciting under the name 'Windows 10 Mobile', of course, unless you get excited by security updates and low level fixes (as I do!) The most intriguing question is whether, when the much-rumoured 'Andromeda' project comes to fruition in 2018, using Snapdragon 835 or better chipsets for a transforming mobile device, any of the existing hardware will be provisioned for any of these builds, just for testing purposes? The jury's out on this, though my guess would be that the Lumias will all be excluded. There's a slim chance that the Elite x3 and IDOL 4 Pro might be allowed a sneak peek - for hardy Fast ring enthusiasts and with disclaimers galore. But don't hold your breath!