Agree with @TheOrangeCrush, this link works. I figured out how to navigate to the Manage Apps page, so I just bookmarked it. They just completely decoupled it from the permissions page, which is where I always found it before.
@papy Thanks for tagging me here, otherwise I wouldn’t have noticed these changes. I do use an integration for bookkeeping, and I have Amazon Pay enabled on my website so that is listed as well. My user permissions page is blank under the Authorized Partners tab. Under the Users tab it has my personal name and if I click on that it says “This is the Primary account manager account for [my brand].” The app/integrations are listed under the Manage Your Apps link that @oneida_books shared–thank you. Everything is working as it should so I don’t want to mess with anything, but good to know where to find this now.
Various eCommerce Industry Newsletters, among other observers, seem to suspect that the policy-changes noted in Ina Steiner’s 030525 Ecommercebytes article “Amazon Sellers Should Check the Status of Their 3P Service Providers” (link) - re: the looming changes in validation/verification protocols with the launch of the SPP (‘Amazonese’ for " Solution Provider Portal" [link, dev-side]) - is most-likely the underpinning factor in what the Amazon Seller Community is seeing, in a variety of aspects, on the User Permissions front.
It’s my belief that Amazon is making this change in error, but we shall see…