Hello,
Installation on debian 10 works now, but on debian 9 it doesn't:
I need to transfer the old database (OpenProject 8) to the new sever (OpenProject 10), therefore i have do upgrade OP first, but the installation on debian 9 doesn't work:

# wget -qO- https://dl.packager.io/srv/opf/openproject/key | apt-key add -
OK
root@erdapfl:/home/rincewind# wget -O /etc/apt/sources.list.d/openproject.list https://dl.packager.io/srv/opf/openproject/stable/11/installer/debian/9.repo
--2021-03-29 00:13:52--  https://dl.packager.io/srv/opf/openproject/stable/11/installer/debian/9.repo
Resolving dl.packager.io (dl.packager.io)... 178.63.71.248
Connecting to dl.packager.io (dl.packager.io)|178.63.71.248|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: unspecified [text/plain]
Saving to: ‘/etc/apt/sources.list.d/openproject.list’

/etc/apt/sources.list.d/openproject.list                        [ <=>                                                                                                                                      ]     120  --.-KB/s    in 0s      

2021-03-29 00:13:52 (2,74 MB/s) - ‘/etc/apt/sources.list.d/openproject.list’ saved [120]


# apt-get update
Hit:1 http://security.debian.org stretch/updates InRelease
Ign:2 http://ftp.debian.org/debian stretch InRelease
Hit:3 http://ftp.debian.org/debian stretch-updates InRelease
Hit:4 http://ftp.debian.org/debian stretch Release 
Ign:5 https://dl.packager.io/srv/deb/opf/openproject-ce/stable/8/debian 9 InRelease
Ign:6 https://dl.packager.io/srv/deb/opf/openproject/stable/11/debian 9 InRelease
Err:7 https://dl.packager.io/srv/deb/opf/openproject-ce/stable/8/debian 9 Release
  410  Gone
Reading package lists... Done   
E: The repository 'https://dl.packager.io/srv/deb/opf/openproject-ce/stable/8/debian 9 Release' does no longer have a Release file.
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.

Hello,

The issue here is related to your old stable/8 URL : "The repository 'https://dl.packager.io/srv/deb/opf/openproject-ce/stable/8/debian 9 Release'"

You can safely ignore that warning and possibly remove the URL from your APT sources list.