As a DD-WRT user since the late 2000's I periodically search this topic. For me the latest "mostly stable" build I found was 33555 (10/20/2017), which I run on my RT-AC66U's and RT-N16's. Shame this seems like it's a dead end.
When I finally replace my routers (looking at the RT-AC86U or RT-AX86U's now) I'll run AsusWRT-Merlin. Downside is it doesn't look like Merlin has all the traffic and bandwidth utilization graphs DD-WRT does, but at least it's current and still being developed.