I think IEXcloud is a new platform that was once part of IEXtrading (Group founded 2012?). I've only used them for a few months for mostly intraday and some historical.
I'll PM you. I have the data you want and will trade for any of a couple of simple programming projects that I haven't had time to do myself. I have 10+ years fairly high quality current names only, 5 years lower quality but including delisted symbols. I have current but not historical SP500 constituents, so you'd have to come up with those yourself. First set includes all pre and post market, second runs 9:00 to 17:00 NYC time.
The JS is simple, you just have to hook all XHR calls by overwriting the prototypes for open and send. I have code for that, you just have to inject it into the target page with Selenium executeScript.
.Thanks for the heads up. They'll certainly monetize at some point. They seem to be associated with Polygon.io (along with at least one other nominally unaffiliated site) so they'll probably follow the same monetization strategy that Polygon followed with its formerly free FX feed.
Check your code on that, you may be doing something wrong. Data appears to go back to 2000. Here is a a URL, using the API key that AndyM so graciously provided earlier in the thread, that returns 3 weeks of AAPL 5min bars from Sept 2000.