Curiousdannii remarked in chat that we have both the tags proprietary-code and closed-source. Their use seems to be very similar. Should we have both, or can we merge them in to one?
An NGram search over google books shows that proprietary code or proprietary software are far more used than closed source.
edit: Maybe throw in proprietary for good measure. There are only two uses (of which one is non-software)