Blix, the developer of the BlueMail email application, claims that Apple copied its anonymous email sign-in feature, named it, "Sign in with Apple," and then removed BlueMail from search results to suit its macOS application.
Blix is not the first company to end up in a situation like this. This problem is called "to be Sherlocked" (a reference to a search program for macOS). It describes a situation when Apple integrates functions that were previously offered by third-party applications into its software products, and then allegedly removes competitors.
In other words, the company first borrows the functionality of programs developed by others, implements them in iOS or macOS, and thereby suppresses them.
For example, macOS Catalina's Sidecar feature that lets you use your iPad as an optional Mac display, was previously offered by third-party Luna Display and Duet apps. There are a lot of similar cases in general; it’s just that not everyone can resist a giant like Apple.
In addition to Blix, another large company called Tile turned to the US congressional antitrust committee this year. Tile is a developer of searchable Bluetooth trackers.
The company requested Apple to open access to the U1 chip, as its tracker and similar devices from other manufacturers may become unusable with the new iPhone. Apparently, the company was scared that Apple was going to release its tracker – the AirTag.
Apple representatives said that the BlueMail application disappeared from the App Store due to security concerns. The company claims to have tried to solve this problem together and return the application. Blix is currently considering filing a class-action lawsuit against Apple.