It's interesting to see in Western Canada a polarization at the provincial levels now.
- BC (while polling is saying otherwise) has always been NDP vs Anti-NDP party (SoCred, BCLibs, United) The greens have always "been there" a goo 10%+ but never been a factor in seats, spoiler yes.
- Alberta was basically a one-party province but is very UCP vs NDP now. There really wasn't a clear anti-UCP choice till 2015, except in the 80s when the NDP then the LIBs took turns winning most of Edmonton. The anti-UCP voter has found a home with the NDP for the most part
- SASK has basically been NDP vs SP for decades, no sign that will change
- MAN is still basically NDP vs PC, where the LIBs used to bump up in seats when the NDP were out of office.
I think most of this started to happen post-1950's
When it comes to BC, it was actually most purely binary in the Bill Bennett years (the combined non-binary opposition adding up to less than 6% in both '79 and '83)--and the most non-binary things got was '91 when BC Libs first overtook the ruling Socreds. Since then it's been binary *seatwise* but there's been a lot more ballot-box oxygen for Greens and other dissident forces.
In Alberta, I'd claim the Libs were very much the 90s clear-opposition-choice equivalent to the NDP today, and in a broader way than just "Edmonton"--particularly after the NDP was wiped off the map in the '93 Klein vs Decore race.
Sask: of course, the Sask Party's only been around since the late 90s. Before that, you had the Libs until the mid-late 70s, then the Tories until the 90s, and then a brief interval w/the Lib parking-spot over the disgraced Tories until the Sask Party was set up.
Manitoba: technically, the NDP *was* in office (but in disgrace) when Carstairsmania struck in '88. Everything thereafter was one continual cycle of back-to-binary "correction" except for the Selinger-twilight blip (so again, it's not so much when the NDP is out of office, than when voters seek to punish the NDP in power)