Are build-deps packages required to stay installed or can we purge them?

I'm often following [SimpleBackportCreation](https://wiki.debian.org/SimpleBackportCreation) to create custom backports for various packages that don't exist in stable branch. One of the steps is to install build-deps with sudo mk-build-deps --install --remove command. The question is, once the backport is installed, do I need to keep build-deps packages installed, or is it safe to purge them once custom backport is installed? I primarily want to save on some space but secondary reason is that updating a backport might uninstall/upgrade previous build-deps from previous version but the build for the new backport version might fail, so I'm wondering if in that case, now removed build-deps packages will affect old backport initially made and already installed. --- Bottom line, can I purge *-build-deps?