Breeze-icons is a freedesktop.org compatible icon theme. It's developed by the KDE Community as part of KDE Frameworks 5 and it's used by default in KDE Plasma 5 and KDE Applications.
The Breeze icons guidelines are described in the HIG section of the KDE Community Wiki.
To report actual bugs and requests for new icons fill a bug in KDE Bugtracker.
You can contact the team through visual-design@kde.org mailing list or Breeze icons' Telegram group.