The only time, IME, that SE will ban a streamer isn't over ACT use in and of itself. The last few bans that I personally have heard about were #1 for use of a mod that gave aoe markers to untelegraphed mechanics during TEA, #2 someone using the Ungermax exploit outside of Squadrons to clear difficult content, #3 a few streamers using ACT and harassing teammates over their dps, and even in that case it wasn't ACT itself that was the issue, but the harassment.