Is Chrome's "this webpage is not found" correct?

I tend to agree with you.

For various reasons 'This Web Page is not found' is not suitable.

I would say 'This web page could not be found'.

My second choice would be 'This web page has not been found'.

'This web page is not found' means almost the same thing but seems to have an air of greater finality about it - almost like saying 'it does not exist'. My suspicion is that it has been designed to suggest such finality, but without going the whole way to a commitment that "it does not exist".

But these are just the finely nuanced interpretations of a native speaker and not necessarily logically implied by the grammar.

You have to accept the validity of past-participles as state descriptors.

• G-d is not found. He is sought.
• The gang leader was not killed that day. The man is not killed. He is alive.
• The house was not painted last week. The house is not painted.
• The room is not dirty. It is not cleaned.
• The child was not vaccinated last week. The child is not vaccinated still.