To toggle features such as `side-by-side` on and off, one solution is to use this shell function:

    delta-toggle() {
        eval "export DELTA_FEATURES='$(-delta-features-toggle $1 | tee /dev/stderr)'"
    }

where `-delta-features-toggle` is this Python script: https://github.com/dandavison/tools/blob/main/python/-delta-features-toggle.

Then

    delta-toggle    # shows current features
    delta-toggle s  # toggles side-by-side
    delta-toggle l  # toggles line-numbers

(It might make sense to add something like this Python script to `delta` itself.)