According to the help center: https://electronics.stackexchange.com/help/on-topic
… if your question …
… is not about …
consumer electronics such as media players, cell phones or smart phones,
except when … modifying their electronics for other uses
… then you’re in the right place to ask your question!
Why then am I getting my question about adding to the feature of a PCB rejected with
Questions on the use of electronic devices are off-topic as this site is intended specifically for questions on electronics design.
Does this only count if I am modifying a water heater for keeping glühwine warm and stirring it at the same time?
In this question Smarting up a water heater I am asking how to add some other sensors to this, modifying it to interface with other systems than those it was designed for. Is it really only on-topic if I was designing a new PCB for it?