It's still there, even if it's not being updated. Anyone can choose to still use it if they wish. And we aren't totally without a game plan, there's already several fresh guides based on rarity, numbers, and factual information being made. (Without considering demand at all, because we all know that is user-based individual opinion anyways)
Since I once followed the now discontinued rares list myself and I'm afraid of how to currently value my older/rarer pets, I will likely not be trading them until we have a reliable trading guide system going again. That's what will make me most comfortable. So for everyone asking how to value their pets now, nobody really knows for sure yet, and that's okay.

I feel that with these new guides coming out, once things settle down and start to take shape, we will have many different, logical systems to create a general understanding of value for everyone who wishes to seek it. But CS users will finally have options. And that is a beautiful thing.
We're all a little uncertain about rarer pet values at the moment, but we have progress to look forward to. Later on, it may hopefully even be easier to trade for pets using new knowledge and factual evidence of why things are valued the way they are. And that sounds way better to me personally. That's just my personal take on this whole situation; maybe it can help someone else feel a little more at ease while everything seems a little chaotic at the moment.