@snowleopard Overall, the building was made in 1906 but the interior was reconstructed in 1999. Apparently there were plans to at least renovate the area and expand just a bit but I'm guessing those have been scrapped.
I'm guessing this video gave the impression that they're inside all day but there's almost no day where they won't be outside. In the winter they let them out at the start of the day's warmest temperature.
The plans to renovate and expand the elephant complex have not been scrapped... This will be the zoo's next project after the expansion and renovation of Gorilla World. Thane has said that the zoo is committed to maintaining elephants long-term, and there has already been a lot of work done to improve husbandry and management.
@GraysonDP - I can't say what it was supposed to have been, but what I can say is that it has fallen through. That said, despite this, the renovation and expansion of the elephant complex is still happening.