Tested on TrueNAS 12.0-U4.
FreeNAS uses /etc/local/sudoers as sudoers file, but this gets overwritten by /conf/base/etc/local/sudoers on reboot.
On TrueNAS 12 /etc/local/sudoers is not overwritten by /conf/base/etc/local/sudoers, so you can't make changes permanent by writing them to /conf/base/etc/local/sudoers.
Instead on TrueNAS 12 /etc/local/sudoers is “rewritten” based on /usr/local/lib/python3.9/site-packages/middlewared/etc_files/local/sudoers.
In addition files that are placed in /conf/base/etc/local/sudoers.d/ are placed in /etc/local/sudoers.d/ after a reboot, but /etc/local/sudoers by default does not include an includedir directive to use files in /etc/local/sudoers.d/.
To make sudoers use an includedir on TrueNAS 12:
echo "@includedir /etc/local/sudoers.d" >> /usr/local/lib/python3.9/site-packages/middlewared/etc_files/local/sudoers
echo "@includedir /etc/local/sudoers.d" >> /etc/local/sudoers
After this it is now possible to place your own files with sudoers lines in both: