Same here. I've sung with "respectable" choral groups and never even heard of this. I suspect it's kinda looked down on by serious musicians. But hey, people like it and that's all that matters. Even in my choruses, they gave us each a practice tape with our parts; so much for snobbery.

Ear training is really difficult if you didn't grow up with it. Some intervals are easy ("Here comes the bride" for a perfect 4th, "My bonny lies over the ocean" for a 6th. There are lots of apps to learn this. If shape notes help, hey.