Pretty old apple tree in the garden which produces hundreds of not-great-tasting apples every year looks like it’s in a bit of a state. I had to cut back a weird branch that had been cut before, presumably because it shot out horizontally, and there’s a ton of rot.
It’s not in danger of hitting any buildings if it ever does fall, although could maybe take out a distracted child or two if I’m lucky.
Is there anything I can do to save it?
Sure. It is really common to see trees planted too deep. If the trunk looks like a telephone pole it is probably planted too deep. The base of the tree should flare out visibly. Unfortunately a lot of trees are too deep when you buy them. This makes tree selection at the nursery very important.
"Mulch volcanos" are a common way you see landscapers apply mulch to trees. The mulch piled up the sides of the tree. When mulching a tree you should use 2"-3" of mulch out to the drip line (edge of the leaves). No deeper than this, and the mulch can't touch the bark of the tree. Wet mulch on the bark encourages fungal growth and pests.
These are both really important for tree health because tree roots need access to oxygen. Being planted too deep or adding too much mulch will starve the roots of oxygen and ultimately lead to tree decline. It won't be a fast death. It could be five years before you start seeing your tree struggle. I have a pamphlet I was gonna take a picture of, but I'm in the middle of a move so it's packed away.