It's the free part. Twilio will also flag patterns like this. We built a contactless workflow that was being censored, and it was due to a similar substring appearing in the link. It was a pain in the ass to figure out
From the Reddit thread and my own testing, links like “ https://free-test.com/” go through fine from a T-Mobile phone. These URLs T-Mobile blocks appear to be individually treated differently, for whatever reason.
Interesting, thanks for checking it out!
Other sites like ddosecrets.com and itsgoingdown.org are also affected. Neither have the string "free" in them