Отработал ли нормально run-user-500.mount?
что показывает "mount | grep run" ?

=8<=========================================================================
root@thinkpad /run/user #systemctl status run-user-500.mount
Unit run-user-500.mount could not be found.
[1]    20030 exit 4     systemctl status run-user-500.mount

root@thinkpad /run/user #mount G run
runfs on /run type tmpfs (rw,relatime,mode=755)
tmpfs on /run/lock type tmpfs (rw,nosuid,nodev,noexec,relatime,size=5120k,mode=755)
runfs on /var/run type tmpfs (rw,relatime,mode=755)

root@thinkpad /run/user #systemctl --type=mount
UNIT                          LOAD   ACTIVE SUB     DESCRIPTION                     
-.mount                       loaded active mounted Root Mount                      
boot.mount                    loaded active mounted /boot                           
dev-hugepages.mount           loaded active mounted Huge Pages File System          
dev-mqueue.mount              loaded active mounted POSIX Message Queue File System 
home.mount                    loaded active mounted /home                           
mnt-nas-doc.mount             loaded active mounted NFS share on nas:/doc           
mnt-nas-music.mount           loaded active mounted NFS share on nas:/music         
mnt-nas-video.mount           loaded active mounted NFS share on nas:/video         
sys-fs-fuse-connections.mount loaded active mounted FUSE Control File System        
sys-kernel-config.mount       loaded active mounted Kernel Configuration File System
sys-kernel-debug.mount        loaded active mounted Kernel Debug File System        
tmp.mount                     loaded active mounted /tmp                            
var-lib-nfs-rpc_pipefs.mount  loaded active mounted RPC Pipe File System            
var-lock.mount                loaded active mounted Lock Directory                  
var-run.mount                 loaded active mounted Runtime Directory               

LOAD   = Reflects whether the unit definition was properly loaded.
ACTIVE = The high-level unit activation state, i.e. generalization of SUB.
SUB    = The low-level unit activation state, values depend on unit type.

15 loaded units listed. Pass --all to see loaded but inactive units, too.
To show all installed unit files use 'systemctl list-unit-files'.

root@thinkpad /run/user #systemctl --state=failed
0 loaded units listed. Pass --all to see loaded but inactive units, too.
To show all installed unit files use 'systemctl list-unit-files'.

root@thinkpad /run/user #cd /etc/systemd/system 
root@thinkpad /etc/systemd/system #ls -lht G user
drwxr-xr-x 2 root root 4,0K июл 15 11:42 multi-user.target.wants
lrwxrwxrwx 1 root root    9 июл 24  2017 user@.service -> /dev/null
=8<=========================================================================

user@.serivice у меня уже не меньше года замаскирован, не думаю что связано.

Возможно связано с user@.serice.
Я не вижу, что отработал logind и pam-systemd.

Размаскировал user@.service:

=8<==========================================================================
#systemctl status run-user-500.mount
● run-user-500.mount - /run/user/500
   Loaded: loaded (/proc/self/mountinfo)
   Active: active (mounted) since Sun 2018-08-19 21:53:07 +07; 4min 9s ago
    Where: /run/user/500
     What: tmpfs
root@thinkpad ~ #mount |grep run
runfs on /run type tmpfs (rw,relatime,mode=755)
tmpfs on /run/lock type tmpfs (rw,nosuid,nodev,noexec,relatime,size=5120k,mode=755)
runfs on /var/run type tmpfs (rw,relatime,mode=755)
tmpfs on /run/user/500 type tmpfs (rw,nosuid,nodev,noexec,relatime,size=795500k,mode=700,uid=500,gid=500)
tmpfs on /var/run/user/500 type tmpfs (rw,nosuid,nodev,noexec,relatime,size=795500k,mode=700,uid=500,gid=500)
=8<==========================================================================

и звук заработал сразу после перезагрузки. Но ведь это успешно работало с 2016-го года, как же так? Есть какие-то комментарии от апстрима на эту тему?

Да, объяснения есть, я оторвал наши предыдущие патчи для работы logind без systemd(под sysv), т.к. это не востребовано. Вместе с этим убрал mkdir из unit файла.

Тогда что, NOTABUG или возвращать mkdir?

кто маскирует user@.serice, тот пусть сам решает возникающие проблемы :)