Reading across several forums, this has become a rather big, mysterious, and sometimes frustrating question.
I've read a few posts where both Sprint and T-Mobile denied they had capability/responsibility of unlocking Sprint phones that meet the Sprint unlocking requirements-- as parts of the Sprint autonomous system get shuttered, or absorbed into T-Mobile's system. The Sprint backbone system just isn't there in it's complete form, or at least not accessible to the individual user, and T-Mobile sometimes says it's not in their bailiwick.
And sometimes later model phones are reported as suddenly becoming T-Mobile eligible, even though they were previously locked to Sprint,without any specific owner request. But in those cases, they seem to remain locked to Sprint/T-Mobile, not unlocked for other networks-- and presumably will now be required to meet T-Mobile's requirements for other-carrier unlocking, instead of the original Sprint unlocking requirements. Or perhaps they'll remain locked to T-Mobile forever, if T-Mobile decides to deny unlocking for phones T-Mobile didn't sell.
For phones that would be useful elsewhere if unlocked, and qualify for Sprint unlocking, I'd follow up now-- and be prepared to file an FCC complaint if you get stonewalled in the process. I don't know what the FCC position is, but it's a route to use if you feel like you're being stonewalled.
For phones that would be useful on T-Mobile's network, but not technically qualifying for unlocking by Sprint rules, I'd try a T-Mobile SIM (doesn't need to be active), and see if that results in an error message. I would be surprised if this extends to AT&T use, though.
The Sprint-locked devices I have are mostly older, and manufacturer-crippled (by lacking bands or GSM radios, or VoLTE capability) for use on other carriers, and I expect them to become bricks, or in my case to see future use as mini-tablets, MP3 players, etc. Their locked/unlocked status is mostly irrelevant, since they won't have a network to connect to.