I know support (from customers) is limited among networks. I know it isn't on all hardware, but does appear to be on at least a couple platforms from the major router vendors. It is supported on an increasing number of DDoS appliances and software packages.
What all networks support receiving BGP FlowSpec information from customers and acting upon it?
There was just a recent discussion about this.
None of the big upstreams support it because they are all too busy selling
their own DDoS mitigation services 
I read that discussion (and several others going back about two or three years) before I posted this.
As an occasional OP on here, I've noticed I get a lot of off-list responses so I obviously wouldn't have seen any of those from other people's threads.
I didn't take that observation away from that thread, but maybe I'm dense. 
I know it was suggested that they wanted to bill for that sued capacity, but that was debunked. I know DDoS services were mentioned, but I didn't see a clear line drawn to that's why it isn't happening... nor confirmed.
Also, what's big? Listed on the Baker's Dozen? Wide-spread POPs on six continents? Showing up on 50 IXPs? 1k IPv4 adjacencies?
A medium sized network that does FlowSpec could be vastly more useful to you than a large network that doesn't.