I don't think they ever got enough yield out of 250nm K6-3 for them to be functional if modded, if there are any around. Seen a lot of decapped K6-2s, have several, all K6-2 shaped,  , no squarer ones. When AMD wanted slower industrial/embedded processors in 2005, they did a run of 500Mhz morgan durons, and the pluses were in that niche also prior to that. 2001 to 2005 AMD had a lot more profitable product to be fabbing, unless they schlepped the 250nm plant to another site, I don't think there was opportunity in there for later 250nm K6-3 cores and nobbled k6-2s to happen, and it didn't happen in 1999, or examples would have been noted among the first wave of K6-2 decappers. When 250nm was done, it seems to have been done, with a lot of the plant recycled into the 180nm that did the pluses and then was too busy doing durons etc.
Unicorn herding operations are proceeding, but all the totes of hens teeth and barrels of rocking horse poop give them plenty of hiding spots.