As the title said where can we find latest developed OS? Developer option in website points to GitHub where I can't find any .iso.
There was a post by the head of elementary on their Medium Blog from 31-Dec 2017 that gives puts forth some information of the upcoming release, Juno. Here is a link:
The last couple of paragraphs talk about Loki being in more of a maintenance mode and that they have "internal alpha quality builds" of Juno.
Please note that this information is now a month old but may shed some light on where the elementary OS Team is in relation to Juno. I'm guessing they will make an announcement when public builds are available.
Hope this helps you out in one way or another but the article is a good read with clear insight to 2017 in review and as well as upcoming events and current activities.