While looking for something else I found that I also have similar messages in my syslog. Looking further I found this recommendation for the vnStat package:
"Every NIC is added on install. So if a NIC is added (or removed) on the firewall, remove the package and install again. If the firewall has data for a NIC vnStat will report the data even if the NIC has been removed.
A reinstall of the package will not change this as the firewall has data pertaining to the non existent data and thus other packages such as vnstat2 will report the data it has or has found."
Link: https://docs.netgate.com/pfsense/en/latest/packages/traffic-totals.html
Indeed I changed my hardware and restored the configuration from the old one. Some NICs changed from 1Gb to 10Gb.
However, after removing and installing the package I see this in the log:
Monitoring (10): tun_wg0 (1000 Mbit) pppoe1 (1000 Mbit) pfsync0 (1000 Mbit) pflog0 (1000 Mbit) ix1 (10000 Mbit) ix0 (10000 Mbit) igb1 (10 Mbit) igb0 (10 Mbit) enc0 (1000 Mbit) em0 (1000 Mbit)
And it's incorrect. My pppoe1 sits on ix0 and my fiber speed is 3/3Gbps, so higher than 1000Mbit vnStat thinks. Also igb0 and igb1 are 1000Mbit, not 10Mbit (they are not assigned yet, though).
I suppose the log messages we see are no more than a minor nuisance but is there a way to assign correct interface speeds for vnStat or disable these messages?