I don’t have links, sorry, but I’ve seen posts on the fediverse from someone who claims to work there and someone who claims to have worked there up until recently.
From them it seems that they are technically still working on it but in a way that it’s clearly either a low priority or something that is now mostly a gesture that is never going to be done. The reason being that Tumblr isn’t in great financial condition right now and once they started thinking about federating it became clear that there’s no financial gain for what would be a decent amount of work.
Basically, it doesn’t seem like there’s any good reason for an independently large and profit-driven social media platform to federate. In the case of Threads, they’re most likely driven by avoiding anti-trust activities.
For-profit entities can work here. Mozilla, Flipboard, Medium seem to all be having a crack. But for-profit social media entities? Dunno, seems like a bust. And if you want a Tumblr like platform here, we might have to build it ourselves (misskey/calckey/firefish might meet your needs in the mean time).