PCLinuxOS was the first fork of Mandriva, originally forked from Mandrake Linux 9.2 back in 2003. But the project has diverged from Mandriva since then, and it hasn't actually been a fork of Mandriva since 2007. PCLinuxOS is its own Linux distribution now. Where Mageia is a recent fork of Mandriva formed by former Mandriva developers, PCLinuxOS is an older fork of Mandrake originally developed by a single person, one that has diverged and moved in its own direction since then.
The OpenMandriva project also still exists. In 2012, Mandriva avoided bankruptcy by spinning off development of their consumer Linux software to the community. The OpenMandriva Association still exists as its own non-profit organization, and has released regular versions since then. But Mageia is more popular and is seeing more active development, so I'd recommend you use Mageia instead of OpenMandriva.
Linux distributions come and go. Sometimes they're ended by a developer who no longer wants to continue working on the project, and sometimes a business goes bankrupt. Other times, the Linux distribution is just left to gather dust and be forgotten. Before Mandriva was fully liquidated, that's what happened to it — Mandriva has actually been gone for years. Now it's just official.
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