Steam developers can now select which Steam for Linux runtime to use for native titles
Steam developers can now select which Steam for Linux runtime to use for native titles

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Steam Client Beta - Steam Client Beta - October 17th - Steam News

Does anyone know what they mean by “legacy runtime environment”? Do they mean running of the host system libraries rather than Valve’s runtimes?
Previously they used library injection and redirection techniques, but the new runtimes are basically containers.
AFAIK yes