Hi, thanks for raising this! Copying from here:
This looks to have been an issue with the HTML index not having been updated. Later versions were still available if you had tried to install though:
$ apt-cache madison librdkafka-dev
librdkafka-dev | 2.8.0-1.cflt~ubu22 | https://packages.confluent.io/clients/deb jammy/main amd64 Packages
librdkafka-dev | 2.6.1-1.cflt~ubu22 | https://packages.confluent.io/clients/deb jammy/main amd64 Packages
librdkafka-dev | 2.6.0-1.cflt~ubu22 | https://packages.confluent.io/clients/deb jammy/main amd64 Packages
librdkafka-dev | 2.5.3-1.cflt~ubu22 | https://packages.confluent.io/clients/deb jammy/main amd64 Packages
librdkafka-dev | 2.5.0-1.cflt~ubu22 | https://packages.confluent.io/clients/deb jammy/main amd64 Packages
librdkafka-dev | 2.4.0-3.cflt~ubu22 | https://packages.confluent.io/clients/deb jammy/main amd64 Packages
librdkafka-dev | 2.4.0-2.cflt~ubu22 | https://packages.confluent.io/clients/deb jammy/main amd64 Packages
librdkafka-dev | 2.4.0-1.cflt~ub22 | https://packages.confluent.io/clients/deb jammy/main amd64 Packages
librdkafka-dev | 1.8.0-1build1 | http://us-east-2.ec2.archive.ubuntu.com/ubuntu jammy/universe amd64 Packages
The index is updated now.