I've been using Tumblr for 17 years. Every one of those 17 years I've seen Tumblr users dramatically declare that the most recent update is the end of the world, and then come back and keep using the site. I've also worked in Community Safety for YouTube, so I'm aware of the type of reports that must be flooding in right now.
I understand why this situation may not seem urgent.
Nevertheless, I do genuinely believe it is. The mood I'm seeing isn't the usual resentment or anger, but rather a defeated acceptance. I have only ever seen this reaction once on this site: after the new adult content rules in 2018. Like a pothole, trust weakens, and weakens, and falls in all at once. Tumblr never got back its reputation from before 2018, and I am afraid that this change will deal a blow from which it will not recover. That is the nature of the resignation I am seeing.
You are asking for feedback on a website that is broken. In fact, the very channel for feedback in which you ask is, currently, broken. The sheer volume of messages explaining the unacceptable costs to your users are invisible. The official announcement that reverting the change will not even be considered for multiple days makes it clear that the organization does not understand the gravity of the situation, or the loss of trust that their inaction is already incurring.
To the small creators who keep the rest of the users on the site, artists and educators and fans and fashionistas, this update is a service outage. It cuts them off from conversation with their audience, the thing they are on this site to do. They've just been told that you have no intention of resuming service.
I have Tumblr Premium because I want to keep playing in this sandbox. This update is a hole in the bottom. Every announcement that insists the app is in a working state is digging it deeper.
This change must be reverted. Not in a few days, after you work out the obvious bugs and go through the reports. It needs to be reverted now. Angry users can be appeased. You can't do that with a userbase that walks away.
I estimate you have 12 hours. Perhaps 24 for the actual reversion update, if your communication team makes it clear you're taking this seriously. You should very seriously consider whether your organizational process can better survive a git rollback, or what will happen without one.
You've consistently fixed usability and security vulnerabilities in hours. You do a thankless job, and you do it much better than anyone thinks you do. I've seen it, and I haven't said it the way I wish I had, but better late than never: It's a good website. My very favorite, even. I met my fiancee here, and we're getting married in 2029, and you can take your credit for that regardless of what comes next.