I am currently unable to install libssl-dev on any of my Debian 11 systems.
Here is a short test based on a public Docker image :

docker run -it --rm --pull always debian:11 /bin/bash
11: Pulling from library/debian
Digest: sha256:6f519a81440354a85eb592c5f32109ab80605f6b892455983a6f618bf87fabe9
Status: Image is up to date for debian:11
root@bd90a8a486f4:/# rm -rf /var/lib/apt/lists/*
root@bd90a8a486f4:/# apt update
Get:1 http://deb.debian.org/debian bullseye InRelease [75.1 kB]
Get:2 http://deb.debian.org/debian-security bullseye-security
InRelease [27.1 kB]
Get:3 http://deb.debian.org/debian bullseye-updates InRelease [44.0 kB]
Get:4 http://deb.debian.org/debian bullseye/main amd64 Packages [8066 kB]
Get:5 http://deb.debian.org/debian-security bullseye-security/main
amd64 Packages [475 kB]
Get:6 http://deb.debian.org/debian bullseye-updates/main amd64
Packages [18.8 kB]
Fetched 8706 kB in 3s (2851 kB/s)
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
All packages are up to date.
root@bd90a8a486f4:/# apt install libssl-dev
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Suggested packages:
  libssl-doc
The following NEW packages will be installed:
  libssl-dev
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 1822 kB of archives.
After this operation, 8188 kB of additional disk space will be used.
Err:1 http://deb.debian.org/debian-security bullseye-security/main
amd64 libssl-dev amd64 1.1.1w-0+deb11u8
  404  Not Found [IP: 151.101.122.132 80]
E: Failed to fetch
http://deb.debian.org/debian-security/pool/updates/main/o/openssl/libssl-dev_1.1.1w-0%2bdeb11u8_amd64.deb
 404  Not Found [IP: 151.101.122.132 80]
E: Unable to fetch some archives, maybe run apt-get update or try with
--fix-missing?

Looking in repo explorer the package is referenced, but the download
link is KO :
- The page does exist :
https://packages.debian.org/bullseye/amd64/libssl-dev/download
- The link is KO (404 File Not Found) :
http://security.debian.org/debian-security/pool/updates/main/o/openssl/libssl-dev_1.1.1w-0+deb11u8_amd64.deb

Same pb when digging into http://deb.debian.org/debian or
http://archive.debian.org/debian and
http://security.debian.org/debian-security : this package seems to be
referenced and reported as dependency, but does not exist neither in
http://security.debian.org/debian-security nor
ttp://archive.debian.org/debian-security (the latter does not serve
Bullseye security-originated packages).

Did I do something wrong ? Is it linked to EOL of Debian 11 ?

Here is a list of packages that give me the same (bad) installation result :
perl-modules-5.32_5.32.1-4%2bdeb11u5_all.deb
libperl5.32_5.32.1-4%2bdeb11u5_amd64.deb
perl_5.32.1-4%2bdeb11u5_amd64.deb
openssl_1.1.1w-0%2bdeb11u8_amd64.deb
ca-certificates_20250419%7edeb12u1%7edeb11u1_all.deb
xz-utils_5.2.5-2.1%7edeb11u2_amd64.deb
dpkg-dev_1.20.14_all.deb
libsqlite3-0_3.34.1-3%2bdeb11u1_amd64.deb

In a VM, starting to install a fresh Debian 11 from the netinst media,
and allowing to access remote repo, turns KO too.

Regards,
Frederic.