Network protocols. They're almost always defined in 7-bit ASCII. I suppose that falls under "legacy systems."
If it makes you feel better, imagine that SMTP's HELO, EHLO, DATA commands are binary. They're all 4 characters long for a reason.
HTTP GET and the URL and all of the HTTP Headers are also in ASCII.
DNS is most definitely ASCII.
Almost all of the network protocols are implemented first in C and intended for very fast processing. The technical reasons to avoid Unicode here are that the communicating processes are really just exchanging raw binary data. The "words" in the protocol are just to make it readable for people with a network sniffer and so that people can do very basic testing using telnet or netcat.
In these situations Unicode conversion is almost always a waste of time.
Pretty much the only place I can think Unicode would be useful for something like web servers and proxies is case insensitive rewrite rules. Case sensitive rules wouldn't matter because UTF-8 matches just fine without decoding.
Personally, I don't believe in case insensitive processing in network servers or file systems. If the request fails to find a resource, bounce it to an error handler script and it can mess around trying to guess at what the user was really after. That keeps things fast and very simple in the common case.
I would ask "Would you define a new network protocol to use Unicode?" except I am afraid I know the answer. I see people writing nasty, nasty JSON and XML "protocols" all the time. And whoever decided to transfer binary data inside XML in Base64 encoding should probably be shot, drawn, quartered, drowned and then buried alive. "Oooh, gigabit networks! I'll just expand everything hugely and make it impossible to use zero-copy!" Bonus points for then compressing the XML for transfer. Bloat it out, pack it down so you can unpack it and unbloat it. While making three copies some of which are expanding the XML and Base64 data into 32-bit UCS-4 "wide characters". Probably in Java so you can use an extra gig of RAM just because.