If you ever use this chimney you should keep the cleanout, that is where all the dead birds wind up.

To close it I would get a few bricks and mortar them in place. In case you ever want to use it the bricks will pop out pretty easily and in the meantime will provide an excellent seal.

I thought the cleanout was where I could sweep in my ashes above and go down and empty it in the basement? We have gas logs...no ask. We also have chimney caps so birds better not get in there?

I was thinking about keeping ot open...who knows we may want a pellet stove or something someday. Right now its lined with fg insulation. The real question is 1) will fg suffice. 2) should I measure exactly where the hole is and put the deminsions some where safe and just drywall over it for now...or keep some sort of little door there for furture access?