That is untrue.  My wife has a grandfathered unlimited SERO plan on Sprint.  It has true unlimited data, never throttled or de-prioritized.  My wife used 300GB during August when she had the little kids with her at the corn stand (Netflix and Amazon).  Have had this plan for over a decade.  Sprint never says a thing.

Cricket is really the best deal.  You can get four lines of unlimited data on Cricket for $100.  No taxes, fees, etc.  It's limited to 3Mbps max, but $25/line is really hard to beat.  It is essentially AT&T service so coverage is usually excellent if AT&T has good coverage in your area.

Cricket is also the only carrier that offers some type of subsidy on phones.  iPhone 7 and 7plus have a $200 discount off retail for purchases.  They are a little older, but the price is excellent. 

I have kids with Cricket and they have no complaints.  No one else can currently match their price. I've looked. 

If you are large enough to have a business account then there are better deals out there.  I work an off farm job and they have 25 lines with AT&T.  They pay $40/month for unlimited.  iPhone 8 64GB is $249.99 with no monthly fees, etc. and they own it.  The consumer is clearly getting bent over on device prices.