[throwaway7033]

The linked Mozilla bug report provides an excellent reason to drop P-521.

If you look at the telemetry for last actual Firefox release, only 8 (yes, eight!) out of 1.67 BILLION handshakes used a P-521 curve. This is a good indication that P-521 isn't needed, at least for certificates verified by web browsers.

[egberts1]

Well, I need it. So, I get to have this. Isn’t the Internet awesome?