XQuartz was always the goto for running X applications on a MacOS desktop. It's been a couple of years since the last version of Quartz and there is no activity on the github, so we may assume that the XQuartz project is dead. Are there any alternatives for running X application on a MacOS desktop?
There are really no strictly better alternatives.
XQuartz is usually the best option for running an X server on a Mac.
If you're running older OS X versions, you might prefer X11.app from Apple.
If you're using older Mac hardware with PowerPC CPUs, you can also run XFree86 in the Darwin/PowerPC port (note that Xorg.org is originally a forked XFree86).
If you simply looking for alternatives - you could also take a look at WeirdX. It is old, it is slow and doesn't have as many features as Xorg - but it is an alternative:
Besides that - there's always the option of running for example Linux in a virtual machine on your Mac, and then run your X server in there (for example Xorg).
Also you could look at alternative ways of connecting to your Linux desktop. For example you could use Xpra:
There's a macOS client - but there's also a HTML5 client for accessing the desktop through a browser.
Another alternative would be to use VNC instead of the X protocol.
UPDATE January 2021: An XQuartz 2.8 beta has just been released with support for Apple Silicon!
"... there is no activity on the github, so we may assume that the XQuartz project is dead."
Reports of the death of Xquartz would appear to be greatly exaggerated: someone has been working on a 2.8.0 release since the start of the year, and there is now a 2.8.0 beta, as a Universal Binary!
and a mail message from the maintainer Jeremy Huddleston Sequoia https://firstname.lastname@example.org/msg01027.html
This release is built with a brand new build system (cf https://github.com/xquartz/xquartz) based on MacPorts + git submodules + our shell script. Older builds required either a lot of hand-holding or Apple Internal tools, so this will hopefully be a step towards making it easier for others to drive future releases of XQuartz.
The Macports version of xorg-server works on Apple Silicon (M1) and replaces Quartz perfectly for me. I can run xterm's and also emacs in x11 mode. The main change I had to make over Quartz was being sure not to set the DISPLAY variable, which can happen in a number of places in the launch or start-up scripts, in my case it was in .bash_profile.
You can use MacPorts. X11 from MacPorts are newer than others.
MacPorts does not use the X11 client libraries provided by the system and installs its own copy. This allows for more consistent support for older machines which are not as up to date (especially Tiger which doesn't have a pkg-config compatible X11 SDK). Most X11 packages in MacPorts are newer than those shipped by Apple and usually reflect the latest work from the XQuartz Project.