dmesg with / without sudo on Debian / Mint

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).