Alright, I think what I am about to send should clarify this. It was taken from a ROBLOX article: https://blog.roblox.com/2016/03/the-future-of-the-roblox-avatar/
They clearly stated the following:
From what I can imagine this means they are unlikely to remove it so you should be able to continue using it in your game. Would like to note this is an article from 2016, but the second article I am about to link below shows their 2018 position on the matter: Roblox Avatar Expansion - Roblox Blog
Hopefully this clarifies everything for you. In conclusion R6 will not be removed anytime soon. As for whether you should use it the choice is yours. Some R6 games are still very popular, so theres nothing wrong with still using it.