Is your feature request related to a problem? Please describe.

No.

Describe the solution you'd like

When receiving messages over the /dev/log unix socket, it would be quite easy for journald to recognize RFC5424 syslog messages (they start with <N>1 rather than <N>Jan.) RFC5424 is much more modern (ISO8601 timestamps, structured key/value data support) than the modified (no hostname field, no linefeed) RFC3164 format previously used for /dev/log.

(For good measure, if RFC5424 is detected, it should probably be fully compatible, including line break handling - classic /dev/log does not expect linebreaks / multiple messages.)

Describe alternatives you've considered

The sd_journal_send family of functions is a systemd-specific alternative to this; however for applications/daemons implementing key/value data it may be difficult/annoying to convert their k/v data to the varargs-based sd_journal_send API. (This is because constructing va_lists in C is only possible through an actual function call - there is no way to build them programmatically. If the k/v data already exists in another format, getting it into a va_list is at the very least extremely annoying if not impossible.)

The systemd version you checked that didn't have the feature you are asking for

Checked Debian's 247.3-3 version and looked at the 247 -> 248 git log & didn't see anything related.