I'm confused about this:

Last minute vote. If the vote is done 5 minutes before the payout then the reward shares are decreased proportionally to the remaining time for the payout. That is, half before 2.5 minutes, and 0 just at time of the payout.

I thought the "last-minute" window was 12 hours, right now, and this wasn't going to change with HF21?!

There is however a 5 minute window with reduced curation rewards right after the post has been made.

Did something get mixed up here or is the "last-minute" really being decreased to 5 minutes?!

P.S.: I checked on github

L27: #define STEEM_UPVOTE_LOCKOUT_SECONDS (60*5) /// 5 minutes
L28: #define STEEM_UPVOTE_LOCKOUT_HF17 (fc::minutes(5))
[...]
L45: #else // IS LIVE STEEM NETWORK
[...]
L61: #define STEEM_UPVOTE_LOCKOUT_SECONDS (60*60*12) /// 12 hours
L62: #define STEEM_UPVOTE_LOCKOUT_HF17 (fc::hours(12))

it's a bit confusing still as it's indeed defined as 5 minutes earlier in the same file, but I'd assume that's a testnet-only setting and the later definition overwrites that.