Let's Encrypt 宣佈 2015/12/03 進入 Public Beta，也就是不需要額外填寫表格申請就可以使用：「Public Beta: December 3, 2015」。
從 2015/09/19 開始到現在已經發出 11K 個 SSL certificate 了：
Our Limited Beta started on September 12, 2015. We’ve issued over 11,000 certificates since then, and this operational experience has given us confidence that our systems are ready for an open Public Beta.
我猜應該還是在 cross sign 取得主流瀏覽器支援後才大量增加的...