People can't even put their for sale posts or wanted posts in the right category, and that's an extremely clear line.
I don't think there's any good reason to split builds based on power levels for legal reasons.
If you wanted to split off posts where noobs want to go 50-100MPH on nonupgraded wallyworld bicycles with a $10 budget, and stick all such threads into their own section so the same things don't have to keep being reposted? I might vote for that.
It might be useful for those looking to emulate high power builds to have an index of them, like Spinningmagnets' various other Sticky Index threads, but that's about all I can think of.
As an example--my builds are perfectly legal, here in AZ: CrazyBike2 is 4kW+, SBCruiser is around 3.5kW, but they are operated under 20MPH. Here there's no power limit, only a speed limit, for electric bikes. (gas bikes have a cylinder size limit). So there's no legal reason to segregate my threads from others with lower power, based solely on power levels. But a rule about splitting would then require mine to move out of the regular bike categories, for no good reason.
For my uses (sole transportation used for heavy cargo/dog hauling, riding in traffic), those power levels are needed to be safe. If I had say, 750W, it would take so long to accelerate up to 20MPH on either one that cars would run me over at traffic lights in the many cases I'm stuck at the front of the line (because often enough, I can't legally go fast enough to make it thru the light and can't just time things right because of other traffic on the road).
CrazyBike2 would barely be able to even reach 20MPH with any headwinds. On SB Cruiser, it's so unaero that it coudln't even reach 20MPH (more like 17-18 at best, assuming zero wind) with only 750W, and if there were any headwinds it'd be bogged down very slow, with some very angry car drivers running me off the road.
Of course, some improvements in aero could help with that, but it'd still be impractical at that point to use them for carrying much in the way of cargo, or hauling the dogs around. Wouldn't be safe to do anymore, based on my previous experiences with lower power levels doing this stuff.