Examples:
Mafuba
This should be a no-brainer, can't defeat the opponent, seal them! However, it was never used again past the 23rd TB. My theory, the problem isn't in the technique but in the container. The enemies got so powerful, that the containers wouldn't be able to hold them.
Dodonpa
It's basically a kamehameha but better. We can assume all of Master Roshi students choose not to try to learn it, because of it's creator and out of respect for their master. Still kinda weird that such a useful technique was forgotten.
Bankoku Bikkuri Shō
You know, the electric like attack Jackie Chun used against Goku in the 21st TB. He called it his most lethal attack, stronger than the Kamehameha. This would be useful in the many battles. Most likely he chose not to teach them.
Shiyōken
Grow two extra arms! This one is weird, it was introduced to be quickly countered. In a way that doesn't seem right, just because one is faster doesn't make having two extra arms redundant. Hum...

There are probably a few more techniques like those. So what do you guys think, what are your theories?