Can' install parrot-sphinx under Ubuntu 20.04 LTS: repo is not signed

Installing parrot by instruction here: https://developer.parrot.com/docs/sphinx/installation.html

However got errors:

$ echo "deb http://plf.parrot.com/sphinx/binary `lsb_release -cs`/" | sudo tee /etc/apt/sources.list.d/sphinx.list

deb http://plf.parrot.com/sphinx/binary focal/

$ sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 508B1AE5
Executing: /tmp/apt-key-gpghome.REPYP0Fez8/gpg.1.sh --keyserver keyserver.ubuntu.com --recv-keys 508B1AE5
gpg: key 47727EDC508B1AE5: public key "Samuel Lekieffre <samuel.lekieffre@parrot.com>" imported
gpg: Total number processed: 1
gpg:               imported: 1

$ sudo apt update
...
Err:5 http://plf.parrot.com/sphinx/binary focal/ InRelease                                                                  
  403  Forbidden [IP: 52.84.109.70 80]
...
Reading package lists... Done
E: Failed to fetch http://plf.parrot.com/sphinx/binary/focal/InRelease  403  Forbidden [IP: 52.84.109.70 80]
E: The repository 'http://plf.parrot.com/sphinx/binary focal/ InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

This solution: Sphinx on ubuntu 19.04 does not work as 20.04 has libboost1.71 while parrot for bionic requires libboost1.65.1 and qt4 which is not available.

Any ideas or wait for release for 20.04?

Need 20.04 however it contains newer packages, so old one does not install.