@vince, thanks for those thoughts. I will add more detail so you don't need to guess on what is likely let alone real...
* Paul Anderson "nailed it" in concluding that my system is running systemd / journald (and journalctl). As I said: "...nailed it. I knew nothing about systemd logging. Now I do..." That was a HUGE "aha!" moment for me!
* As of March 13 when I started this thread, I hadn't custom configured any aspect of logging. (I since did discover the page you linked, and now I do have a logrotate-enabled weewxd.log file ;)... but that's immaterial to the original question.)
* This VM (in ProxMox 7 VE fwiw) is running a straight install of Ubuntu 20.04 LTS (and weewx 4.10.2, not that it matters AFAIK?).
* I had no idea my setup is unusual. That's exactly why I asked the question. Until this situation, I was under the impression that Ubuntu is derived from, and essentially similar to, Debian. I've got several Debian VM's but thought I'd try something more stable, hence my choice to use Ubuntu LTS. As I'm sure you know better than me, the weewx install for Ubuntu is literally identical to that for Debian. All of my other Linux instances (VM, NAS, and other) have what I consider a typical logging setup (various files in /var/log). So I expected this to function exactly the same. Not knowing there was something different, how would I even suspect something about the logging system???
A suggestion: apparently systemd, and journald/journalctl, are the up-and-coming 'new' logging methodology for linux distros (only around since 2015 LOL!) It may help to eventually mention `journalctl` and/or other associated commands in one or more of the documentation areas, eg FAQ, User's Guide, etc. (As of this moment, the following google searches find no mention: site:
weewx.com journalctl ... or site:
github.com/weewx journalctl)
(BTW, the github record shows that weewx itself must understand the os enough to know where to look, and how logging works. There was a `journald` related patch just last year, and quite a few systemd-related items. If weewx needs updating for compatibility, I'm not surprised that the docs might need updating as well.)