`<backtab>` is called a "function key", but it can be bound to a command in any keymap. (Forget about `function-key-map` in this context.)

Vanilla Emacs (`emacs -Q`, no init file) binds some function keys in various keymaps. And libraries you load can do so too.

Work from the modes that are current at a given time, in any context. Use `C-h k` to find out what such a key is bound to. Determine what keymap it is bound in (probably a mode keymap), and then unbind it from that map (in your init file, on the mode hook, for example).

You can see the keys that are currently bound in a given keymap using `C-h M-k` (`describe-keymap`) followed by a variable bound to a keymap (or followed by an actual keymap, if there is no variable). You need library [`help-fns+.el`](https://www.emacswiki.org/emacs/download/help-fns%2b.el) if you want to use `describe-keymap`.

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(But I do wonder why, other than for learning about keys and keymaps (which is a good thing), you are undertaking this exercise of removing key bindings. More typical is simply binding a different key to the command whose binding you don't like. Anyway, I don't need to understand why.)