I was an admin on a forum for a sports management game (Championship Manager /Football Manager) for ~20 years and for us the 'death' came from Steam Community/Workshop, Reddit, Twitter, and Discord.

Visitor numbers always remained pretty high (People coming to the site to download resources etc.) but people were just less likely to contribute with posts as they had other places to use as a community instead.

Here's the number of posts for each yearly release of the game...

  - 2009: 110,000 posts
  - 2010: 273,000 posts
  - 2011: 280,000 posts
  - 2012: 188,000 posts
  - 2013: 196,000 posts
  - 2014: 180,000 posts
  - 2015: 159,000 posts
  - 2016:  96,000 posts
  - 2017:  50,000 posts
  - 2018:  32,000 posts
  - 2019:  21,000 posts
  - 2020:  24,000 posts
  - 2021:  17,000 posts