iPhone 6 iOS 11.4.1 Jailbreak MIA
If you want to keep your iPhone 6's jailbreak don't install iOS 11.4.1.
Developers have released a working iOS 11 jailbreak, but it only covers iOS 11.0-iOS 11.1.2. It doesn't support iOS 11.2, iOS 11.2.1, iOS 11.2.2, iOS 11.2.5, iOS 11.2.6, iOS 11.3, iOS 11.3.1, iOS 11.4, or iOS 11.4.1. Not yet at least.
The current iOS 11 jailbreak doesn't come with Cydia, but that could also change down the road.
Unfortunately, there's no way to downgrade to any of Apple's older iOS 11 updates. If you already moved to iOS 11.4.1 or one of the newer versions, you're stuck until developers release a new jailbreak. There is some good news though.
It looks like a researcher successfully cracked Apple's iOS 11.3 update though it's unclear if the jailbreak will be made public. There's a very good chance it'll stay private.
Keep an eye out for updates as we push into 2018.