So, on April 6, Tweak Developer @ijapija00 has posted a video on Twitter on which he demonstrated an iOS 10.2.1 Jailbreak being applied on iPhone 7. He called it YUCCA Jailbreak after his tweak / stasher, YUCCA Utilities. A lot of people went ahead and liked his post (we're talking ~1000 people) due to the credibility he has built due to the article wrote by The Redmond Pie about his so called "Mobile Substrate Fix" for iOS 10, that targeted iOS 10.x Jailbreak (Yalu Beta 1) released by Luca Todesco. Unfortunately, the fix he made worked, but bricked some of the devices, or put them in a condition that would make the next Betas very hard to apply, thus making Luca Todesco's life pretty hard. The Mobile Substrate was disabled by Todesco on purpose so that the next Betas could be applied without having to deal with people that install various tweaks, like OpenSSH.
Despite being a tweak creator and knowing, at least to some extent how iOS works internally, @ijapija00's Jailbreak demos never ended up in a release. He has demoed in the past a lot of "jailbreaks" for iOS 10.1, iOS 10.2, iOS 10.2.1, iOS 9.3.5, iOS 10.3, etc, but no release.
This particular iOS 10.2.1 Jailbreak for iPhone 7 looked pretty real, until Miguel, from iDevice Help spotted something abnormal, the Emoji selector panel on ijapija00's iPhone 7 has transparent icons instead of white filled ones specific to iOS 10.2.1, this clearly hints to the fact that he is using iOS 10.1.1 or lower. Yes, there are chances that he uses an emoji tweak, but there aren't any tweaks for the icon filling itself.
ijapija00's jailbreak demo: https://goo.gl/DFydND
Pangu Jailbreak Team at MOSEC 2017 | Pangu is Back?
How to Start iOS Hacking: https://youtu.be/hy81mnLMvnE