P-521 is used at run time in 50 crates (of which 3 optionally). It is a direct run-time dependency in 9 crates. It's used only as a dev dependency in 2 crates.

Depender (with downloads and own rev deps) P-521 version
36K 9 russh-keys ^0.13
49K 29 ssh-key optional ^0.13.3
13K 19 deno_crypto ^0.13.3
800 1 tangy-lib >=0.13.3
4 openpgp-x509-sequoia ^0.13
1 openpgp-pkcs11-sequoia ^0.13
clevis >=0.13.3
galileo-osnma optional ^0.13
1 jaws optional ^0.13
12K 5 vsss-rs dev ^0.13
10K 1 voprf dev ^0.13.3