this will trigger them asE_USER_ERROR. IMO, it is too high. What aboutE_USER_NOTICE instead ? Lower than notice would be even better as this is a debugging info, but trigger_error does not have something lower (as it is not meant to be used as a debug logger)

btw, I'm not even convinced this is a good idea (at least not today as people will complain about silenced notices in the profiler)

looking at the phdoc, the default isE_USER_NOTICE, so all good.
but I now made it explicit, and configurable (via a third "$level" arg)

hum OK, I removed the trigger_error call for now - but kept the $level arg: even unused, it make the signature compatible with future changes requiring it.

I don't think passing a level here makes much sense, especially not as a PHP error level. It would couple it too much to the error triggering layer, while we talk about logging here.
If you want to provide a logging level, use PSR-3 levels instead.

Btw, a way to allow flexibility about adding a level only the day we need it is to make the method final: final method can add optional args as they want (adding an optional arg only breaks BC for inheritance)

good idea, they are final now