iPhone 6 iOS 12.5.2 Jailbreak
You can jailbreak newer versions of iOS 12 using one of the latest jailbreak tools though you'll want to avoid iOS 12.5.2 for now.
The developers behind the "unc0ver" jailbreaking tool for iOS have released a version that's compatible with older versions of iOS like iOS 12. There's also another jailbreak tool (checkra1n) that works with older software.
Unfortunately, it's unclear if these tools will support iOS 12.5.2. If you're jailbroken, or want to jailbreak, you'll want to stay put on older iOS software.