There is any evidence the "new" Windows Mobile will be extended also to a 2015 smartphone as the 950 is.
From what I have seen (windows code, such as the composable shell code, where win10m is listed along side the new "Andromeda"), and heard (insider rumours call it a "branch", not a "reboot" of win10m), "the new" windows mobile isn't a replacement OS/hardware its more of a variation, or sibling.
From what I can tell the new device is multi-screen, and so the UI is likely quite different, and not particularly applicable to standard slab style devices.
However in so far as the 950's support goes, that's a complex story. You'll hear a hundred versions of this.
ATM it gets support based on it being one of the few recent phones in circulation, and 2015 isn't _that_ long ago.
Once their are a significant number of newer models, it will likely be cut however. But there's not a lot of new phones on the obvious horizon, other than perhaps an x3 refresh, and perhaps this Andromeda maybe next year, so I think for now support is okay, IMO.
The other issue is feature2. MSFT has been focusing on bugfixes primarily whilst it does other work in other areas on the onecore code - they told us, this was temporary and for the purpose of these one core changes. In light of windows on arm, windows s, the new console SKU, and this Andromeda, this kind of makes sense - there's a bit of a core code shuffle for sure.
But in the insiders team, it was recently stated, there is no immediate plan to take it off feature2, yet. But at the same time, we have been promised some enterprise features, recently, and during build they talked of some new features coming to windows 10 mobile, and its also been officially confirmed they are changing the keyboard to one based on 'swiftkey' across all of windows.
So there will certainly be more bugfixes, and some new features, but the exact timing and nature of which is pretty uncertain, and a bit murky.
If I had to guess, to completely stab at the dark, I'd say this year will include a few small feature updates (the enterprise ones, maybe something like files on demand), and we might get a few more in the early next year update (such as the promised timelime, the new keyboard), but probably by fall next year, there will be newer phones, and past that the 950 might not get support.
That's my guess based "the feel" of where things are at right now with mobile, and commercial considerations. But it is a guess.
Other people might be more pessimistic, or more optimistic in their assessment, truth is we are all guessing. Support does only get phases out however when there is something to replace it, in circulation, and there is no such thing right now, nor imminent.
At build when they demo'd files on demand, they showed it on a Lumia 950, whilst explaining things like timeline etc were coming to windows 10 mobile.
But you should be aware that, as an older phone, things going right for msft, its not going to be supported forever.
Just as importantly however, the OS is fairly feature rich compared to other mobile operating systems. Inline replies for example is only _just_ coming to android next version. Systemwide dark mode, has been recently poorly imitated with an inversion method in recent ios and android. Widgets on android are hopelessly unreliable, and the widget screen on ios (new), is off the home page, so you have to navigate away to see it.
So long as MSFT's UWP plans unfold as they plan, and more apps trickle into the platform, you don't realistically have to worry about new features, even if none of the above came. None of them are particularly must have IMO.
In the past, when support has been dropped for other handsets, such as the CU, MSFT extended a bit of an olive leaf, and allowed insiders preview to still get the update. While this won't mean you get this multi-screen OS (wouldn't make sense anyway), if there are new updates to win10m that aren't rolled out to the 950, because there are new slab phones like the elite x3 refresh, it may still be possible to get those updates, provided they run okay on the device.
Nobody can really tell you exactly what to expect, because mfst is keeping its cards close to its chest atm. But if you are just coming to win10m, like I have, you will likely enjoy the 950 for some time before feeling the need for a new phone IMO - because the screen is good, the camera is good, its a nice looking phone, and coming from BB there are more apps than either amazon app store, or BBW, and here win10m benefits a little from its sibling OS windows 10.
The one thing I am fairly confident about, is that "windows on arm", will encourage more native UWP app development, for a variety of reasons. So for me, that will be fun to get the spin off, from.
Consider also that the 950 can be had for 200-250 USD or so, if you shop around, but whatever new phones come out will be closer to 700-1000 on release IMO. Even if the support doesn't last long, its not like you are burning money - your getting a recent-ish premium decent phone for a very reasonable price, when the newer replacements potentially on the horizon will be new, very premium handsets with prices reflective of that. In view of that, if there is a new sibling OS, or newer feature updates to newly released phones, its probably worth waiting for the price to sink anyway.