I thought I would do some research.
- Twitch bought Curse Inc August 16, 2016
- The Twitch App beta, which integrated addon support from Curse, launched March 16, 2017
- Full changeover from Curse Client v5 to the Twitch App occurred August 10, 2017
That means that everybody had anywhere between four and 16 months' notice of the Curse Client v5's eventual demise. The replacement Twitch App may not have existed until March of this year, but if anyone applied logical thought to Twitch buying Curse, it was inevitable that a new client would be developed, simply because Twitch would want integration of its services into a singular app that also provided Curse's services.