openssl in nixpkgs is openssl 3. See NixOS Search
openssl 1.1 is also available. See NixOS Search
However, openssl 1.1 is EOL and in nixpkgs it is marked as such. You’ll need to explicitly specify that you’ll allow this package. Inside config = {} you’ll have to set permittedInsecurePackages = ["openssl-1.1.x"] so that Nix doesn’t complain about the package being insecure.
Apparently asdf only includes a precompiled version of python that depends on openssl 1.1. You could also consider using GitHub - cachix/nixpkgs-python: All Python versions, kept up-to-date on hourly basis using Nix., which also holds most python versions, like asdf, but built using Nix. From what I could tell these packages use openssl 3.