According to https://packages.debian.org/source/sid/isc-dhcp
the Git repository of the package is on Salsa. However, looking
at the repository, it has been inactive for more than 2 years
and the latest patches are not part of it. See:
https://salsa.debian.org/dhcp-team/isc-dhcp/activity

Using `apt-get source` does provide the latest patches; using
a mixture of Salsa and package repository.

```
root@bullseye:~# apt-get source isc-dhcp
Reading package lists... Done
NOTICE: 'isc-dhcp' packaging is maintained in the 'Git' version control system at:
https://salsa.debian.org/dhcp-team/isc-dhcp.git
Please use:
git clone https://salsa.debian.org/dhcp-team/isc-dhcp.git
to retrieve the latest (possibly unreleased) updates to the package.
Need to get 1281 kB of source archives.
Get:1 http://deb.debian.org/debian bullseye/main isc-dhcp 4.4.1-2.3 (dsc) [2684 B]
Get:2 http://deb.debian.org/debian bullseye/main isc-dhcp 4.4.1-2.3 (tar) [1190 kB]
Get:3 http://deb.debian.org/debian bullseye/main isc-dhcp 4.4.1-2.3 (diff) [88.1 kB]
Fetched 1281 kB in 0s (2985 kB/s)
dpkg-source: info: extracting isc-dhcp in isc-dhcp-4.4.1
dpkg-source: info: unpacking isc-dhcp_4.4.1.orig.tar.gz
dpkg-source: info: unpacking isc-dhcp_4.4.1-2.3.debian.tar.xz
dpkg-source: info: using patch list from debian/patches/series
dpkg-source: info: applying dhclient-script-exit-status.patch
dpkg-source: info: applying dhclient-dividebyzero.patch
dpkg-source: info: applying dhclient-release.patch
dpkg-source: info: applying dhcrelay-listen.patch
dpkg-source: info: applying dhcpd-conf.patch
dpkg-source: info: applying fix-exit-hook-manpage.patch
dpkg-source: info: applying fix-manpage-macro.patch
dpkg-source: info: applying fix-spelling.patch
dpkg-source: info: applying disable-nsupdate.patch
dpkg-source: info: applying system-bind.patch
dpkg-source: info: applying bind-includes.patch
dpkg-source: info: applying configure.patch
dpkg-source: info: applying Fixed_gcc_10_compilation_issues.patch
dpkg-source: info: applying 4.4.2.CVE-2021-25217.patch
W: Download is performed unsandboxed as root as file 'isc-dhcp_4.4.1-2.3.dsc' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)
```

Cheers

Dear submitter,

as the package isc-dhcp has just been removed from the Debian archive
unstable we hereby close the associated bug reports.  We are sorry
that we couldn't deal with your issue properly.

For details on the removal, please see https://bugs.debian.org/1143544

The version of this package that was in Debian prior to this removal
can still be found using https://snapshot.debian.org/.

Please note that the changes have been done on the master archive and
will not propagate to any mirrors until the next dinstall run at the
earliest.

This message was generated automatically; if you believe that there is
a problem with it please contact the archive administrators by mailing
ftpmaster@ftp-master.debian.org.

Debian distribution maintenance software
pp.
Thorsten Alteholz (the ftpmaster behind the curtain)