There are several previous threads answering this question of a "historical" Arthur, like this very comprehensive one from /u/Whoosier addressing the available historical sources that mention Arthur. I think /u/bitparity summed it up best when he said:

As Guy Halsall pointed out in his recent book on the subject, Arthur may have actually existed, but the almost complete lack of documentary evidence means there is nothing we can usefully say about him historically.