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.)