I’m a Polish national working remotely as a freelancer for a US company. Would I be in violation of eTA rules if I continued working during my visit to Canada? Do I need to declare it? (Assume that my trip will take place after all COVID-related restrictions are lifted)
This is allowed. Canada is unusual in adopting such a modern approach to the question.
The government of Canada has a page discussing the definition of work in the context of work visas, which explicitly lists as an example of an activity that is "not considered to be 'work'"
long distance (by telephone or Internet) work done by a temporary resident whose employer is outside Canada and who is remunerated from outside Canada;
I would not declare it unless you are asked a question that can't be answered straightforwardly without mentioning it, or unless you're planning such a long visit that the question is inevitable. (Exactly how long that is probably depends on your circumstances.)