Just updated BetterTouchTool to V2.645 and found that the accessibility permission warning popup cannot be dismissed as the close icon of the popup panel is now missing. Actually BTT can work well for touch/click functions without accessibility permissions.
After some searching around, one github issue has some useful hint:
defaults write com.hegenberg.BetterTouchTool BTTDisableAcccessibilityCheck YES
Only valid for High Sierra and previous OS X versions, Mojave will force to enable the accessibility for BTT.