Supercell has never been ambiguous about this. In their , they explicitly ban:
A is an unofficial version of the game hosted on non-Supercell servers. These servers typically use emulators—such as the Ultrapowa Boom Beach Server (UBS) —to replicate the game's environment while providing players with modified permissions. Private Server Boom Beach
You will only encounter NPC (Non-Player Character) bases and events like Dr. T or Colonel Gearheart. 🛡️ Official Safety & Reporting Supercell has never been ambiguous about this