If I understand correctly, the signatures generated by PuTTY aren’t perfectly random, so if someone got a hold of a bunch of keys from a server, they could figure out the pattern. It takes about 60 keys. This affects not just PuTTY, but also FileZilla, WinSCP, TortoiseGit, and TortoiseSVN.
In other words if you have NIST P-521 keys, or any others using 521-bit ECDSA, you should revoke them and generate new key pairs. After you update your software.
I remember before scrambling they just put blocks that prevented you from going to certain channels. I somehow figured out if you ran the cable box through the VCR first and put it on channel 2 while the TV was still on 3, it would shift all the channels down one. Cinemax was channel 14, which our box just would not go to. But it would go to 13, so doing my little trick teenage me got to watch a lot of skinamax.