I think there is no such keyword in GnuPG
2.1.x source code. _pgpkey-http only can be
found in GnuPG 1.4.

For version 2.1.15

root@47b54eb8e5bb:~/gnupg-2.1.15# gpg2 --version
gpg (GnuPG) 2.1.15
libgcrypt 1.7.3-beta
Copyright (C) 2016 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later https://gnu.org/licenses/gpl.html
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Home: /root/.gnupg
Supported algorithms:
Pubkey: RSA, ELG, DSA, ECDH, ECDSA, EDDSA
Cipher: IDEA, 3DES, CAST5, BLOWFISH, AES, AES192, AES256, TWOFISH,

CAMELLIA128, CAMELLIA192, CAMELLIA256

Hash: SHA1, RIPEMD160, SHA256, SHA384, SHA512, SHA224
Compression: Uncompressed, ZIP, ZLIB, BZIP2
root@47b54eb8e5bb:~/gnupg-2.1.15# gpg-connect-agent --dirmngr 'getinfo dnsinfo' /bye
OK - System resolver w/o Tor support
root@47b54eb8e5bb:~/gnupg-2.1.15# gpg2 --keyserver hkp://t1.zhsj.me --recv-keys 7DFBB2F2
gpg: keyserver receive failed: End of file
root@47b54eb8e5bb:~/gnupg-2.1.15# gpg2 --keyserver hkp://t2.zhsj.me --recv-keys 7DFBB2F2
gpg: keyserver receive failed: No keyserver available

For version 2.1.11
zsj@debian ~ $ gpg2 --version
gpg (GnuPG) 2.1.11
libgcrypt 1.7.3-beta
Copyright (C) 2016 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later http://gnu.org/licenses/gpl.html
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Home: ~/.gnupg
Supported algorithms:
Pubkey: RSA, ELG, DSA, ECDH, ECDSA, EDDSA
Cipher: IDEA, 3DES, CAST5, BLOWFISH, AES, AES192, AES256, TWOFISH,

CAMELLIA128, CAMELLIA192, CAMELLIA256

Hash: SHA1, RIPEMD160, SHA256, SHA384, SHA512, SHA224
Compression: Uncompressed, ZIP, ZLIB, BZIP2
zsj@debian ~ $ gpg-connect-agent --dirmngr 'getinfo dnsinfo' /bye
OK - System resolver w/o Tor support
zsj@debian ~ $ gpg2 --keyserver hkp://t1.zhsj.me --recv-keys 7DFBB2F2
gpg: key 7DFBB2F2: "Shengjing Zhu <i@zhsj.me>" not changed
gpg: Total number processed: 1
gpg: unchanged: 1
zsj@debian ~ $ gpg2 --keyserver hkp://t2.zhsj.me --recv-keys 7DFBB2F2
gpg: keyserver receive failed: No keyserver available
zsj@debian ~ $