Mar 18 05:12:01 Turris_JB syslog-ng[3418]: WARNING: experimental behaviors of the future syslog-ng 4.0 are now enabled. This mode of operation is meant to solicit feedback and allow the evaluation of the new features. USE THIS MODE AT YOUR OWN RISK, please share feedback via GitHub, Gitter.im or email to the authors; config-version='4.0'
#############################################################################
# OpenWrt syslog-ng.conf specific file
# which collects all local logs into a single file called /var/log/messages.
# More details about these settings can be found here:
# https://www.syslog-ng.com/technical-documents/list/syslog-ng-open-source-edition
@version: 4.0
...
Yep, @lucenera was right about it. I activated those new features as default, so anyone could try that before, there is going to be a switch to a newer version, which happened in Turris OS 6.3.3. Well, actually, it happened to OpenWrt as I cherry-picked it there.