Debian machine with "jessie-backports" no longer have release file

Today I get this message when trying to run apt update on my Debian:

Hit:1 http://security.debian.org stretch/updates InRelease
Ign:2 http://mirrors.digitalocean.com/debian stretch InRelease
Hit:3 http://mirrors.digitalocean.com/debian stretch-updates InRelease
Hit:4 http://mirrors.digitalocean.com/debian stretch Release                                            
Ign:5 http://ftp.debian.org/debian jessie-backports InRelease                                           
Err:6 http://ftp.debian.org/debian jessie-backports Release                         
  404  Not Found [IP: 130.89.148.12 80]
Get:7 http://deb.goaccess.io stretch InRelease [2,520 B]                     
Get:8 https://packages.sury.org/php stretch InRelease [6,760 B]              
Hit:9 https://download.docker.com/linux/debian stretch InRelease
Get:10 https://deb.nodesource.com/node_6.x stretch InRelease [4,635 B]
Hit:11 https://repos.sonar.digitalocean.com/apt main InRelease
Err:8 https://packages.sury.org/php stretch InRelease      
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY B188E2B695BD4743
Reading package lists... Done 
E: The repository 'http://ftp.debian.org/debian jessie-backports 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.
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://packages.sury.org/php stretch InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY B188E2B695BD4743

Any help will be much appreciated :)