No one is teaching 1HB anymore. Along the way, I went to several tennis schools, and it's quite hard to find a coach that offers training as a 1HB, most of the time because the person is a 2HB himself or herself or does not know what it takes to train a 1HB.
The training itself is tougher. I had a coach who said that for every person that chose 1HB, he demanded at least 600 shots in the tennis wall in comparison with 200 of the 2HB, and with the game being as complex as it is in several other disciplines (footwork, volley, baseline, physical training), the least thing that you can do is to have almost 1 hour extra of training for a single shot.
With the game becoming slower, 1HB became a honeypot for lefty players who could explore it a lot and baseline players who would return every single stroke. When people used to play on the carpet, one average player serving an American twist or a flat on the line made it very hard for a 2BH to return due to the speed and anticipation needed. Today we have 2HB returning 150 MPH serves.