https://www.prepaidphonenews.com/2019/01/boost-mobile-will-only-support-3g.html
This does not surprise me at all. Realistically-speaking, there are probably very few as a percentage of actual Boost customers using 3G-only devices, and they'd be better suited towards an LTE or VoLTE device anyway. This will get rid of a lot of "backup backup" phones for Reddit and Nth phone junkies, but I think this is a necessary step as we move towards LTE as the "fallback" standard.
I wonder if VMU will have this problem, too.
Still no word from actual Sprint on this, but I'll be following this very closely for obvious reasons.
Well, they're not all 'backup' phones. There's a continuing demand (although small as a percentage of users) for both long-battery-life flip phones, and likely a smaller demand for qwerty phones and smaller-screen Android devices: and the existing stock of older devices makes up most of the options. Sprint, and to a lesser extent AT&T, seem to be the last refuge for these users. I do realize the move to LTE is inevitable.
LiterallyUnlimited, could you keep us posted if you hear more about how this might affect Sprint-network users outside of Boost?
Reading between the lines of the linked post, I had wondered if this was a similar situation to the demise of single-frequency Sprint-network phones, where frequency re-farming made it impossible for some phones/locations to receive PRL updates (or initial PRL provisioning.) ..... or if it's just because Boost has always preferred to limit the range of allowable devices.
From what I understand, there newer flip phone that support VoLTE, have a notably lesser battery life than the older flip phones.
That has been the case when LTE does came out... Battery life shorter when using LTE vs 3G.
Aside from Alcatel GO FLIP, is there currently any other Sprint flip phone that may support VoLTE?
https://www.sprint.com/en/shop/cell-phones/alcatel-go-flip.html
This Kyocera has 4G and LTE, but I do not know about VoLTE.
Thanks. I don't see LTE listed in specs on Kyocera website: DuraXTP - Kyocera Mobile
This Kyocera model ("candybar?") does have LTE in specs: Kyocera DuraTR, the rugged LTE feature phone
My bad for not seeing the false claims in the Ebay ad. The bar phone looks good, but at $325 from Sprint---no thanks. Perhaps a used one at a reasonable price can be had.
Kyocera DuraXV LTE is available for Verizon, so perhaps Sprint variant will be available in the near future.
"It's also powered by a 1,530 mAh battery, which should provide up to 7.7 hours of usage time or up to 18 days of standby time."
According to Phonearena.com specs, supports VoLTE:
Kyocera DuraXV LTE specs
Verizon also has the Kyocera Cadence, a more traditional (not Mil-Spec) flip phone, that retails for $120. No Sprint version yet, but as Sprint rolls out more VoLTE, there could be one.
It's not too hard to find used Kyocera Dura models, and they are affordable, if not cheap. The extreme build quality makes used ones a fairly safe bet.
This year I think we'll be seeing the release of new LTE based flip phones supporting VoLTE. Flip phones are still in demand in a certain market segment. And with the carriers moving away from 3G support there will be a demand for VoLTE flips. The above Alcatel is such an example.
I would assume they'll have stronger batteries to support the increased power required to support 4G networks.
From what I've gathered on this, the only affected devices are going to be WiMAX (which, if you're still using a WiMAX device in 2019, you really should upgrade) and older 2G or 3G-only phones, like 10-year-old Blackberrys or Palm Pres. I don't believe any device that has LTE connectivity is affected at this time.
See: OMA Slot Devices
Thanks, LiterallyUnlimited!
Unfortunately, for non-mainstream users, there are a lot of problem-solver devices on that list.
Most of the Kyocera Dura series, as well as the Sanyo (Kyocera) Pro 700 flip phone. (Probably other flips, too, that I don't recognize.)
Nearly all of the qwerty-keyboard phones, both slider and bar. (I've kept at least one of these activated as my outdoor jobsite phone.) At least one isn't on the list (the LG Mach), so maybe I can switch to that one.
Some of these are not surprising, since they were on the similar list for problematic PRL updates a couple of years ago.
I love my Kyocera DuraMax 
In the same vein, I love my Kyocera DuraXT and my very old mother in law loves her Kyocera DuraPlus. If Sprint drops 3G, then "Dura" has lost its meaning.
Here's a site that will be providing info on what MNVOs may be impacted by Sprint's present and future 3G shutdown. There's little reported at this point, but at least there is an attempt to get info.
For Boom... 
Sonim XP3 rugged flip phone for sale on sprint.com.
https://www.sprint.com/en/shop/cell-phones/sonim-xp3.html
According to specs, supports LTE. Uncertain about VoLTE.
ETA: AMR-NB and AMR-WB audio codecs in specs, which may indicate support for VoLTE.