Babies are complicated. Toddlers, almost as much. I no longer have the free time I once did. I do plan to update the guide now that I'm playing FFXIV again (even if only at night), but it's far from a simple update, and I do not have a handle on the current economy. If you (or anyone else) are offering to help, that would be great. If not, it'll be updated slowly over time.
There is also not currently a strong reason to do desynths like there used to be in 2.x. You can break even on some items, but there are no longer any new desynth-only items, either glamour items like the old swimsuits and fish minion or crafting mats like the FC/BC/MC demis. Desynth has been seriously neglected even as they have made significant changes to ease leveling it. That makes it hard to be motivated to do a guide - I can, and will, do a leveling guide, but I don't currently have an answer to "why should I level desynth" beyond "because then you'll have higher level desynth."
I will be updating the LTW stuff first, from my own experiences. It's the only desynth I'm currently working on, personally. I don't know if I'll update the 1-100 leveling guides - things have changed, but I don't think enough to warrant a change to those. If someone wants to help with the leveling guides for other classes, please let me know.
You can never desynth the latest equipment. Verity items will be desynthable when the next set of tomestones comes out (and primals when the next set of primals comes out as Nixxe said).
EDIT: According to the 4.0 patch notes, desynth success rates have changed again. If anyone can give me a few datapoints to add to my own, it would be immensely helpful! Please give the name of the item desynthed, your chance to desynth it, your dSkill before desynthing it, and your dSkill gained from the desynth (if successful). I don't need many datapoints, just a few scattered ones, especially at the earlier levels. I mainly need to use them to confirm my equations.