The first public iOS 13 jailbreak is still at least a few months away. However, many developers from the jailbreaking community have already made it clear that it is very much possible to jailbreak iOS 13.
Building further on that, @Pwn20wnd confirmed last week that Cydia works just fine on a pre-jailbroken iPhone running iOS 13. This should not be surprising since Cydia itself is just an app that will work as long as the necessary patches are there. While Cydia works, some minor modifications are required to ensure that Substrate also works under iOS 13.
If anyone's wondering, I can confirm that Cydia works fully fine without any changes on iOS 13.0 :-). (Tested on a pre-jailbroken installation with @CorelliumHQ)
— cy:pwn20wnd (@Pwn20wnd) July 11, 2019
Cydia is the most popular package manager in the jailbreaking community. One of the reasons behind its popularity was the lack of rivals and its wide developer support. However, with the release of Chimera jailbreak and Sileo package manager earlier this year, things are no longer the same.
Saurik is very much frustrated with maintaining and updating Cydia for every new iOS device or jailbreak release and just wants to leave the entire thing behind. This is also the reason why Cydia looks so dated and just barely about works. In comparison, Sileo offers a modern UI and it is actively being worked upon as well.
Here’s hoping that whenever the first iOS 13 jailbreak drops, it relies on Sileo as its default package manager instead of Cydia. It would do a world of good to all the parties involved.