The marketplace switched to OpenVSX in mid-2025. Here’s the announcement post: Extension Marketplace Changes - Transition to OpenVSX. The extension itself was probably never on OpenVSX.
If it was working for you recently, it was likely still installed locally from before the switch, or you installed it via VSIX at some point. Once you remove it or move to a new environment like a devcontainer, it can’t be downloaded again from the marketplace because it isn’t published there.