Maintenance of iris surround pond

Well, it's obviously a mature pond with masses of what looks like Flag Iris around it. Irises foliage will die back some during the Winter, but some will remain. It won't do any harm to the Iris to thin it out but unless you really want to tidy the pond up I wouldn't cut back the foliage drastically as it's obviously still getting energy from the Sun to store in its rhizomes for flowering next year. I never cut back my Iris in my wildlife pond and only thin them when I need to.
To be honest, not knowing the real depth of the pond apart from being told it might be "extremely deep" is, from the safety side, not good for carrying out any maintenance on the pond. I would strongly advise that if you know any one who coarse freshwater fishes to ask them to use a float and weight to plumb the depths. It can be done very easily and quickly and if he/she plumbs the length and width of the pond you will have a plan of the pond depth.