As there is now the possibility of adding a note to a question, should not be there a note for questions that have been asked in the past, and that would be closed, if asked nowadays?
For example, the following note appears in some questions on Stack Overflow:
This question exists because it has historical significance, but it is not considered a good, on-topic question for this site, so please do not use it as evidence that you can ask similar questions here. More info: FAQ.
The alternative is to close those questions that are now considered off-topic, or that would be closed for any other reason. If there are questions that we think worth being on EL&U, we should consider adding such note.
For a list of possible candidates, you can search them using this search: is:question answers:20 wiki:1 closed:0.