The flavor text on Shadow Sliver tells us it was one of the last Slivers that could claim direct lineage from the Sliver Queen, my question is, which are the other ones we know about?
The others on Dominaria are results of Jurassic Park-like experiments by the Riptide Project on Otaria, they used some Sliver fossils they found (implied to be transplanted on Dominaria through the overlay with Rath). They didn't know about the Sliver Queen and her necessity for a working Sliver hivemind though, so most of them were dangerous and feral after their recreation. This in combination with their high adaptability caused the slivers to overrun the Riptide labs and infest first Otaria, then the rest of Dominaria. Only recently did they start evolving a hivemind that works without a Sliver Queen. The slivers on Shandalar are a different story, no one knows where they are from, they could even have evolved there. The slivers in Modern Horizon may or may not be extant on other worlds than the ones we know of (Alara and New Phyrexia), but that's just speculation.