Valve ported Source to 64-bit Windows in 2005 (https://vghe.html/source-engine.html) but there were bugs, and it seems like they just let it fall by the wayside (I don't think it's been available since 2006, maybe 2007 at the latest). And yeah, it seems like they've been abandoning Source 1 for a while now. Valve recently hired Joshua Ashton who has been giving Team Fortress some love but it seems like the focus is well and truly on Source 2 now.