You can easily install it manually post-install, so there is little point in adding it to the Calamares installer.
Here is how you do it:
https://wiki.archlinux.org/title/Deepin_Desktop_Environment
The problem with adding it to the installer is the fact that sooner or later someone will blame EndeavourOS for Deepin being buggy and unusable.1 Most of the people who install EndeavourOS expect it to work, and that’s not an entirely unreasonable expectation, to be honest…
It’s just bad optics to include it. Deepin is for foo , adventurous advanced users, and they do not need a GUI installer to install it. Especially not on a terminal-centric distro 
1 If you think that’s unrealistic and that I am overreacting, just keep in mind that there are people who think that a text editor crashing is a problem with EndeavourOS ISO, and it took dozens of posts to convince them that this was obviously not true. Imagine someone like that installing Deepin DE from the ISO… The unavoidable (yet still false) conclusion would be that EndeavourOS sucks.