It is still a rumor and should be treated as that, but AMD might be introducing XT versions of some of their processors in the Ryzen 3000 product family. There's been some chatter about a refresh in the line for a while now, in the June timeframe for an announcement.
If that happens then it looks to be a June announcement for an XT lineup of processors. And yes, the XT suffix you have seen and know from the Radeon product lines, ergo this sounds quite plausible for a more premium SKU. Three products would be launched, the Ryzen 9 3900XT, the Ryzen 7 3800XT, and the Ryzen 5 3600 XT. The products all have the same physical specifications in core counts; 3900XT has 12-core/24-threads; 3800XT has 8-cores/16-threads and the 3600XT 6-cores/12-threads part.
So what's different then? Well, perhaps AMD had a peek at Intel's PL2 power states, and figured we can do that too? Long boosts with likely a bump in processor frequency.
Plausibility? GIGABYTE referenced a "Matisse Refresh" in its motherboard product roadmaps. 16 June would see announcements with availability on July 7th. It is also rumored that the new XT processors take the current pricing position and that the other processors will drop in price. For now it remains a rumor though, and let's hope it's not just OEM only.
The clocks are now leaked as well. If the rumor is true then Ryzen 9 3900XT would get a boost clock of 4.8 GHz, and that is 200 MHz more than the 3900X model. Importantly though the base clock would be also be modified to 4.1 GHz and that is a 300 MHz faster than the existing model, these would be significant changes:
RYZEN 9 3900XT - 4.1GHz base, 4.8GHz boost.
RYZEN 7 3800XT - 4.2GHz base, 4.7GHz boost.
RYZEN 5 3600XT - 4.0GHz base, 4.7GHz boost.
RYZEN 9 3900XT - +300MHz base, +200MHz boost.
RYZEN 7 3800XT - +300MHz base, +200MHz boost.
RYZEN 5 3600XT - +200MHz base, +300MHz boost.
And now benchmarks have leaked at various sources (TUM_APISAK (Twitter). Tum Apisak, shared what seem to be system details from 3DMark FireStrike benchmark runs. Apisak's findings have since been followed up by Twitter's RoGame.