Hot openers

Just been watching Mania 1, which opens with Tito Santana squashing The Executioner

These days now most openers tend to be much longer and more competitive, with the idea being to start hot

When did this approach roughly start coming into effect?

Would you say it started with Midnights Vs Southern Boys at GAB 90?

​No, because that wasn't the opening match.  
That was really an innovation of sorts from WCW, though, with the prototype being Jushin Liger v. Brian Pillman at Superbrawl II, I'd say.  Previously, the thought was that you didn't want a hot opener, because then no one could follow and you'd kill the crowd.  ​But then the lucha libre and Japan junior influences started creeping in and it became a thing.