Looks like TestFlight's S3 bucket, covering the period before and up to when Apple acquired the company, was indexed and scraped, and has leaked to archive.org.

This likely contains every unencrypted binary submitted between 2012 and 2015 to the service — ‘terabytes of data’. Dated February 2015. “…and includes several unreleased Angry Birds games.”

https://www.eurogamer.net/apple-testflight-servers-from-2012-to-2015-leak-containing-terabytes-of-data

Apple TestFlight servers from 2012 to 2015 leak, containing terabytes of data

Data scraped from the servers for Apple's TestFlight service circa 2012 to 2015 have been leaked, giving access to tens…

Eurogamer.net
It might take a very long time for anybody to trawl it and find worthwhile applications, but I expect most of them will be for arm32 and not run on modern devices. You will likely also find apps and features that never made it to the App Store, either having been discontinued during development or rejected outright by App Review. Honestly a goldmine for historical preservation — and I hope it can remain public
@stroughtonsmith someone needs to find Flappy Bird.
@stroughtonsmith oh geez. Well I’ll have a few things in there. Good to know /sigh