The Tumblr staff account was already posting weird updates about making Tumblr more like Twitter, basically, but they've stepped it up in the past few weeks to the extent that they had to reassure everyone that they're not getting rid of the reverse chronological feed, really! Truly!

But it doesn't sound good: https://staff.tumblr.com/post/722477242948747264/tumblrs-core-product-strategy