The same thing has happened to Fyreslayers - it may be that at last we will get some more units, as while Seraphon need their Saurus to be more tanky and Killy, Fyreslayers have strong enough units as they are, but they need more diversity.
In fact, it looks as though all pre-Sylvaneth Battletomes have been removed from the website, meaning that it’s likely that all those armies may well get new Battletomes apart from Stormcast, who already have theirs, and Khorne Bloodbound, who are now merged with the Khorne Daemons to form Blades of Khorne who have a post-Sylvaneth book. Whether these will be before Moonclan and Darkoath, after or mixed in between, it looks uncertain at the moment, but it is certainly good news.
Certainly those armies can be divided into those who just need updated rules and some plastic updates of old fine cast minis (Seraphon, Flesh-Eater Courts) and those who need a bigger selection of models and updated rules (Fyreslayers, Bonesplitterz, Skaven Pestilens), in addition to those armies that exist and still don’t have tomes (Dispossessed, Gutbusters, Brayherd). I look forward to seeing what they do with these older armies anyway.