A brief history of guitar strings and bending technique
Back in the 1930's and 40's before rock and roll was invented, guitar strings were very thick and heavy. Because of this, jazz and blues guitar players alike were limited to slight string bends or half bends. Although this greatly developed the strength in blues and jazz players' fingers, it definitely limited their technique and freedom to try new types of bends.
It could also be said that this lack of technique due to heavy guitar strings was a blessing! Jazz and blues players alike figured out other ways to coerce new sounds from the guitar. Jazz players like Django Rheinhart and Charlie Christian and blues players like Tampa Red and Blind Lemon Jefferson used hammer-ons, pull-offs, slides, vibratos, and double notes to create new guitar licks and riffs that are still used today!
As guitar string technology improved in the 1950's and 1960's, blues and rock players began to push the limits of string bending technique. Guitar technique would never be the same thanks to Chuck Berry, Buddy Guy, Eric Clapton, Jimi Hendrix, Keith Richards, and many others.