That was 2023, and one of very few things made not to specifically promote their hardware or as a cheap spinoff of existing IP. And define "actively maintaining", because general bug fixes for decade old multi-player games and managing item marketplaces doesn't require much manpower.

Going further back there's Aperture Desk Job which was a tech demo for the Steam Deck in 2022. Then an extended cut version of Artifact originally meant as a sequel in 2021, which is a Dota 2 card game, but still remains unfinished, so effectively abandoned. Then Half Life: Alyx in 2020 which 90% of gamers can't play because it's VR only, and clearly made to further promote their VR hardware. Dota Warlords in 2020 which was originally a community game mode. The original Artifact in 2018, which had abandoned iOS and Android ports. The Lab in 2016 which was made to promote the launch of the HTC Vive. A zombie CS spinoff in 2014, Dota 2 in 2013, CS:Go in 2012, Portal 2 in 2011, and Left 4 Dead 2 in 2009.

If you remove the spinoff and niche stuff from the list you get game releases in 2023, 2020 (arguable since it's VR only and thus inherently niche), 2013, 2012, 2011, 2009.

That's a pretty big gap of not much for the last decade game-wise. Its been previously documented and published that Valve has issues getting games developed because of the flat organization structure. Articles like this.