Natural Selection 2
Natural Selection 2 is a sequal to Natural Selection and was originally announced on 31st October 2006 (https://unknownworlds.com/ns/news/2006/10new_site_old_max_ns_is_four_and_announcing_ns2). The first media release was on 1st December 2006 and showed dynamic infestation in a map but no game-play (McGuire, https://unknownworlds.com/ns2/news/2006/12/dynamic_infestation). Some concept art, screenshots of some Lua and some information about development tools was released periodically during 2007 and 2008 as development continued.
On 10th July 2008 Unknown Worlds announced that they were switching from the Source engine to an in-house engine (Cleveland, http://unknownworlds.com/ns/news/2008/07/unknown_worlds_podcast_26). Reportedly the Unknown Worlds team wanted to continue licensing the Source engine tooling but Valve refused to license the tools without the engine.
It is unclear how progressed the game was before it switched away from the Source engine: a development video of a map viewer application showed just one non-development map: ns2_tharum though it never existed in the game's source control (https://discord.com/channels/84689542480674816/361232765460676610/1143360195280523274). A number of .vmf files existed but they were not fleshed out maps (https://discord.com/channels/84689542480674816/361232765460676610/1143366105629458554).
Natural Selection 2 was eventually released in 2012 using the Spark engine.