I just installed 20.2 Manjaro Gnome. The GUI does not work. boinc.service is missing as well.
/ >>> boincmgr  06:46:32: Warning: Mismatch between the program and library build versions detected. The library used 3.0 (wchar_t,compiler with C++ ABI 1013,wx containers,compatible with 2.8), and your program used 3.0 (wchar_t,compiler with C++ ABI 1014,wx containers,compatible with 2.8).
Well, I would include a screen shot here but someone has stopped us from using things that would actually help.
I run boinc on quite a few machines having used many distros over the years. It appears the packaging still isn’t quite right in the ARCH family of distros.