I've been doing some boot-to-root VMs lately and some of the classics are quite old, with Kioptrix: Level 1 going all the way back to 2010. While the puzzles and exploits still hold up, it's not always easy to get hold of tooling - especially newer stuff like "socat" - for these older distros.
For that reason I've compiled a bunch of variants of busybox, netcat, and socat for different architectures and distro's (mostly older Debian) and put them up on github: https://github.com/hypn/misc-binaries. While you definitely shouldn't download and run executable files a stranger told you to, they will likely help you upgrade your reverse shells in VMs should you choose to use them.
I like the idea of "statically compiled" binaries, but they come with a bigger memory footprint and require more diskspace which can make them harder to upload (and in some cases, apparently, can be less portable due to conflicts of libraries)... so I've produced both statically and dynamically compiled files. The repo contains both files compiled as per my recent blog post on compiling binaries for different architectures, but many more compiled in VMs to ensure they not only work for the architectures but also older Linux versions (thanks QEMU!).
As it turns out it's also not entirely easy or straight forward compiling these apps from source in older kernels, but with some hacking and patching I'm fairly happy with what I got to compile. Hopefully the effort will be worth the reward.