Tumblr's change in policy on NSFW content was bad enough, but what made it a complete disaster was outsourcing the enforcement of that policy to a crude image classifier. A lot of non-pornographic content got removed when that happened, and a lot of users never bothered contesting those removals (either because it was too much effort, or because they were no longer maintaining their account). So a lot of content on older Tumblr accounts is just gone.