I've read that OpenGL was deprecated in macOS Mojave 10.14 in the favour of Metal, Apple's proprietary low-level graphics API. Has it been completely removed in macOS Catalina? glxinfo appears to be not installed.
OpenGL is still supported in the latest iMac - supported versions
If it has been removed, is there a way to install it?
OpenGL is officially deprecated by Apple starting with macOS Mojave 10.14. However, Apple is still maintaining the standard.
OpenGL and OpenCL
OpenGL and OpenCL were officially deprecated in Mojave last year, though that's a little misleading since it implies that Apple had been actively maintaining and updating its support for those standards. In Catalina, as in every macOS version going all the way back to Mavericks, the macOS OpenGL implementation is stuck at version 4.1 (2010), and the OpenCL version is stuck at 1.2 (2011). This means that apps that still rely on those APIs on macOS will continue to run, provided they've been updated to meet the 64-bit-only requirement. But you shouldn't be developing new Mac apps that rely on OpenGL or CL for anything important.
The situation can be misleading for OpenGL developers. But Apple is clearly hinting they will not support the technology moving forward in the future.
It is still there and located at /System/Library/Frameworks/OpenGL.framework but it seems glxinfo is missing
See comment on original question for information on installing glxinfo
glxinfois not a general OpenGL tool. It's specific to the X Window System, which is not the native windowing system of macOS. If you install XQuartz or X.org, it should come withglxinfo.