Please fix "affect specular" option on Lights with Shadow Catcher/matte material.
Currently all 2024/2025 and 2026.x releases have this BUG. Last one when it worked is 2023.1.3.
You can add another texture environment and plug it in to the "visible environment" of render target node (and use checkmarks there you want) like "Backplate visible".
Thats OLD Octane, it was not removed in that version, i was under impression you used newer version LW/Octane i.e. 2023+
That older LW (2020) + Octane 2021.1.7 has both options for Alpha, Premultiplied (and straight when it's off) in settings. It's checkmark on imager node: here: https://prnt.sc ...