Just bumping this thread to say I’ve been emailed by SNCF to say that the Carte Avantage is again at 50% off 
https://www.sncf-connect.com/app/catalogue/description/carte-avantage-adulte
You have up to 5 months from the date of purchase to activate it, so may still be useful if you’re not planning on using the train much immediately.
Edit: although the Carte Avantage Senior is the more interesting one to get if you’re 60 or over. Otherwise it seems you have to stay overnight for the discount to be applied.