Site not down but not usable at the moment.
Here is my unbiased analysis to current chipmixer's situation (time will tell if this is true):
1) Attack against Chipmixer isn't all that volumetrically significant.
2) Chipmixer had been watched and targeted by state level adversaries with assistance by deep state and related big data entities can do mass surveillance on hosting providers /CDN entites with AWS/Google/Cloudflare user data exfiltration to facilitate necessary traffic and data correlation attack.
3) major transit providers to datacenter hosting clear web probably now had span port enabled for quite site time on their switch to log, spoof and identify the .onion site's origin. This isn't likely to change.
4a) DDoS is necessary to attempt to trick browser to leak information related to chipmixer session key over TLS/SSL (that can be restored / steal privatekey and chips) but since chipmixer uses minimal .js was the likelihood of success in this side-channel SSL content scrubbing trick isn't all that great.
4b) sustained DDoS would be necessary since it would compel site under DDoS (unfortunately for the adversaries Chipmixer isn't clueless) to reconsider use DDoS CDN services like Cloudflare.
4c) Once Cloudflare or similar CDN is used, all chips, sessions, and private keys can be monitored and logged by deep state, since now the privatekey is available for the SSL Cert (on *.cloudflare domains) used to serve up and fetch content proxy to chipmixer web site. This is done with special SSL pinning appliance with cloudflare's private key tapping (span) Chipmixer scrubbed/clear traffic channel and exfiltrate IP, User Log (session), and even private key and bitcoin address of all transactions or activities involved.
5) Chipmixer Admin realize the above situation and quietly re-architect the server infrastructure to parallel a dozen of .onion mirror site with different Host/transit ISP to survive similar deep state surveillance attack in the future. How much help Chipmixer from expert in this area forum and resources such as bitcointalk would likely determine how soon site returns to normal service.