I realized recently that I haven’t gotten any alerts from LibreNMS recently, including when I rebooted devices for patching. After going to the “Alert Transport”, and attempting to send a message I got “SNMP Error: Could not authenticate.” Others seem to recently get this as well. (Link)
Turns out after May 31st (although for me it seems more like June 6th, 2022) Google disabled simple password logins for Gmail accounts. You need to enable two factor auth, then enable an app specific password for LibreNMS. This was a good quick guide on how to do that. With LibreNMS sending alerts when something is wrong, but not having a alert that it is working, it may be worth going and checking if you use LibreNMS and Gmail.