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I noticed this difference some time ago, but until now, I didn't bother to ask, why is that?
On Linux Mint 18 (Ubuntu-based) I can run dmesg without using sudo.
On GNU/Linux Debian 9 I must use sudo in order to use for example dmesg.
I wonder, where is this behavior coded? And can it be changed?
This is controlled by the dmesg_restrict sysctl entry, documented in the kernel documentation. Its default value is determined by the CONFIG_SECURITY_DMESG_RESTRICT kernel configuration value, which is typically enabled in modern distributions.
You can see the current value by running
/sbin/sysctl kernel.dmesg_restrict
and change its value using (as root)
sysctl -w kernel.dmesg_restrict=1
(to enable the restriction) or
sysctl -w kernel.dmesg_restrict=0
(to disable it and restore the old behaviour).
To make this change permanent (automatically applied at boot), write it to /etc/sysctl.conf or a configuration file under /etc/sysctl.d:
echo kernel.dmesg_restrict=0 | sudo tee -a /etc/sysctl.d/99-dmesg.conf
It is the default kernel config of newer kernels: CONFIG_SECURITY_DMESG_RESTRICT=y
As others have answered you can use /sbin/sysctl to inspect and change values at runtime.
However, to make these changes persistent across reboots you should add them to /etc/sysctl.conf
