I have never heard of them before. They do not believe Jesus is God and also believe in a flat earth. Anyone else heard of them? Is this a new group or one that been around for awhile? I figured some flat earthers might have heard of them.
I actually had heard of them before. They've been around for a while (since at least 2005, which is how far archive.org has their website) and started out as a weird variant of Seventh Day Adventism. However, a number of their beliefs have shifted from what they had originally. For example, I don't think they were promoting flat Earth until around 2016.
Back in 2007, someone discusses them here:
Group says John Paul II to return as anti-Christ [speroforum.com] The World's Last Chance believes that they have found clues within scripture that su...
www.byzcath.org
This has some useful, if dated, information on them, and most importantly talks about the belief that was the main thing World's Last Chance focused on: That John Paul II would appear to revive from the dead, but it would actually be a demon impersonator. This would be hailed as a miracle and the impostor John Paul II would become the pope after Benedict XVI, and would be the last pope and this would lead into the end times.
The above link doesn't discuss much in the way of the reasons World's Last Chance claimed this, but this belief was hinged on their belief that the kings in Revelation 17:9-11 were popes starting with Pius XI (I think they started the count with him because of the Lateran Treaty?), and thus the eighth and final king/pope would be after Benedict (the seventh pope/king), and the mention of how the eighth would be one of the original seven means it was one of the earlier popes. This is the basis for the idea that John Paul II would be fake-resurrected to become the eighth pope and bring about the end times.
So they viewed the end times as imminent, which is presumably where the "world's last chance" came from (it's the world's last chance before all this bad stuff happens). They had other beliefs (rejection of the Trinity being one, as you mention) but this end times belief appears to have been their main focus, and certainly was what differentiated them from others who otherwise had some similar beliefs.
Of course, as everyone knows, this whole "impostor John Paul II" thing never happened, and after Benedict XVI resigned, Francis--who never claimed to be any kind of resurrected John Paul II--became pope in 2013. It appears they changed their belief on the subject to saying that maybe the impostor John Paul II thing wouldn't happen, but it would still be the pope after Benedict who was the last one and would cause worldwide persecution and issue in the end times and continued to promote the idea that the end of the world was imminent. Then in 2024 it looks like they abandoned all of these eschatological beliefs and switched to something similar to partial preterism (the belief that Revelation was mostly fulfilled in the first century), which is why you won't find articles on their site advancing the end times stuff discussed; after renouncing the idea, they deleted all of them.
As noted, the "flat Earth" appears to be a belief they came into later, but with them abandoning their original main message, they're probably leaning into things like that more now.