Ways to find Single+Multi games on Steam:

• steamdb.info/tag/3859/ — "Multiplayer"
• co-optimus.com/games.php
  • search for "PC games"

Tags found on steampowered.com itself:

• single/multi + Casual
  • single/multi + Casual + Relaxing
    • single/multi + Casual + Relaxing + Atmospheric
• single-multi + Family Friendly + Relaxing

Tags found on steampowered.com, but excluding the Singleplayer tag (because the VAST majority of multiplayer games also have a single-player mode):

• Multiplayer + Casual
  • Multiplayer + Casual + Relaxing
    • Multiplayer + Casual + Relaxing + Atmospheric
• Multiplayer + Family Friendly
  • Multiplayer + Family Friendly + Relaxing