Cydia is one of the applications used to jailbreak the iPhone. It’s pretty popular and used by a lot of people, until Apple updated iOS with tighter security and made it harder for people to make modifications. Update after update of iOS is also getting longer, Apple added many features and functions that used to only be used with jailbreak.
Even so, a jailbreak for the iPhone is actually never dead. Only its use is limited and it is a bit tedious, for example if already using a jailbreak and the device is dead - the jailbreak needs to be done again.
Odyssey Team which is the developer company that produced the jailbreak system for iOS 14 with the code name Taurine is now rumored they will come up with apps and systems to jailbreak devices running iOS 15. This includes with iPads running iPadOS 15. The system is named Cheyote and it can only used on iOS 15.0 to iOS 15.1.1. Support for iOS 15.4.1 will also be released later, but it will take more time to develop.
Those of you who are interested can follow the progress of the Odyssey Team on Twitter to see when the system will be offered to the public. Just is it worth it to jailbreak the iPhone? To use it you can't update iOS, which if an older version has a security vulnerability - you'll risk your data and device just like that.