I can confirm that placing example.jinja in /srv/pillar makes it found. But that doesn’t really solve the problem when the goal is to use /srv/user_pillar.
Additionally, adding /srv/user_pillar doesn’t have any visible effect (?) - even after restarting.
# /etc/salt/master.d/user.conf
pillar_roots:
base:
- /srv/pillar
user:
- /srv/user_pillar