Yes, the countdown is becoming more common pratice among anyone experienced in Feast, be it JP, NA or EU.

JP also use "Hello" as a signal for Ranged DPS to their Melee friends that their burst is ready. Sadly in EU or NA Hello outside of the first one at the beginning of a match is considered condescending so the community is trying to determine what the 'Tell' may be. Even though it may confuse people with the Countdown macro, I tend to use "Everyone Attack". So far it still proves effective. If my allies are not using Countdowns (Typically the Melee DPS or the Tank if they're particularly aggressive are supposed to use them.) Ill hit the countdown just to give them indication of timing. Doesn't always work but when it does it can make a match fairly clean.

As a point of feedback a simple "Ready!" would be great to have as a quick-chat - hopefully that may be part of the upcoming adjustments as its a simple fix.