Is 涼台 not used anymore and does it differ from 陽台？
Both 陽台 and 涼台 can be translated as 'veranda, balcony and deck' 

I suppose 陽台 refers to a deck you can sit on to enjoy sunshine, and 涼台 is a deck you sit on for cooling down. Which means the location of this deck determines how you call it.  

陽台 should locate in an area that is open and have access to plenty of sunshine. While 涼台 should locate in an area with shade. surrounded by tree, or near water. 

陽台 is a more common term because most people live in cities don't have a backyard to build a deck for cooling, but most apartment buildings do have balconies.