If you want the latest pflogsumm release form unstable on your Debian trixie/stable mailserver you've to rely on pining (Hint for the future: Starting with apt 3.1 there is a new Include and Exclude option for your sources.list).

For trixie you've to use e.g.:

$ cat /etc/apt/sources.list.d/unstable.sources
Types: deb
URIs: http://deb.debian.org/debian
Suites: unstable 
Components: main
#This will work with apt 3.1 or later:
#Include: pflogsumm
Signed-By: /usr/share/keyrings/debian-archive-keyring.pgp

$ cat /etc/apt/preferences.d/pflogsumm-unstable.pref 
Package: pflogsumm
Pin: release a=unstable
Pin-Priority: 950

Package: *
Pin: release a=unstable
Pin-Priority: 50

Should result in:

$ apt-cache policy pflogsumm
pflogsumm:
  Installed: (none)
  Candidate: 1.1.14-1
  Version table:
     1.1.14-1 950
        50 http://deb.debian.org/debian unstable/main amd64 Packages
     1.1.5-8 500
       500 http://deb.debian.org/debian trixie/main amd64 Packages

Why would you want to do that?

Beside of some new features and improvements in the newer releases, the pflogsumm version in stable has an issue with parsing the timestamps generated by postfix itself when you write to a file via maillog_file. Since the Debian default setup uses logging to stdout and writing out to /var/log/mail.log via rsyslog, I never invested time to fix that case. But since Jim picked up pflogsumm development in 2025 that was fixed in pflogsumm 1.1.6. Bug is #1129958, originally reported in #1068425 Since it's an arch:all package you can just pick from unstable, I don't think it's a good candidate for backports, and just fetching the fixed version from unstable is a compromise for those who run into that issue.