Maybe not in the latest upgrades, but AWACS planes still use klystron vacuum tubes for the power amplifier of their radar transmitter.
Somebody mentioned the uTracer as a modern alternative. I've actually bought one.
It may lack the power and higher voltage for the larger tubes. But it is completely adequate for preamp/small signal tubes (12AX7, EF86 etc.) or the "smaller" power tubes (cf. EL84).
I've bought mainly because I've some interest in guitar/bass amps, and the DIY building aspect was the fun part of it.
However, compared to the 570, it has quite a few shortcomings.
The heater supply is not good (square wave with filtering but without regulation).
The DIY aspect can be fun for some but a big problem for people wanting something ready to use.
In particular, the kit is basically a bare PCB, with no enclosure or even a tube socket.
The UI is a computer program, which is good. However, interfaces with lots of knobs, one per function instead of sub-menus, are more hands on experience.
So I can understand why people would prefer something like a 570. It's just too bad that prices are not-reasonnable.
By the way, even with the mentioned shortcomings, the uTracer is very nice and I'm happy with it.