If you consider the 15 W TDP of this CPU we assume that Slack is using no more than .075 Watts of power.I'll accept that. You've conceded that it's a "tax" of about a tenth of a watt from the entire user base. It's not about saving the planet. It's about the fact that one kind of expense is being saved while another kind of expense is being shifted to the user base. Some people might well find it a worthwhile tradeoff. The market certainly has spoken on this matter.
How much extra energy does allocated RAM use vs. unallocated RAM?
That's the wrong question with regards to RAM. With regards to RAM, it's more about about a kind of opportunity cost. (Capacity cost?) It's not relevant to casual users who have a top of the line machine just to run their web browser and email. However, it is very relevant for someone with any kind of intensive use case. "Ounces add up to pounds, which add up to pain." It's just much more convenient for the user if their machine stays responsive even when you're doing something intensive.
Turning your screen brightness down, banning Electron, or running Lynx instead of Chrome isn't going to save the planet, even if all 7 billion people in the world did it.
It's not about saving the planet. It's about following the "money," even if the amounts are minuscule. That said, if the Electron community got together and put together some technology that reduced their collective footprint, that could only benefit them and their users.