I want to be able to do something like this to be able to export only valid, non-expired encryption-capable subkeys:
gpg --export-filter 'drop-subkey=expired -t || revoked -t || usage_flag !~ e'
what's missing is the `usage_flag` option for the `drop-subkey` filter type for `--export-filter`.
This doesn't seem to be implemented yet, though [Werner mentioned it over a year ago on gnupg-users](https://lists.gnupg.org/pipermail/gnupg-users/2017-March/057835.html) as a desirable outcome:
> This is just a first step; we also need a properties for the key capability.