The basic idea has existed since the beginning of typography, but I think the specifics are new.
If you want to typeset text, you need to adjust the horizontal space between adjacent letters. For example “AV” needs to have the V overlapping the A, but “AL” does not.
With moveable type, the physical letters had to fit together properly, so that they could be clamped into a frame containing a whole page of type. Therefore, there were different versions of a letter like “A” fixed onto different sized blocks of wood or metal, and the person assembling the page of type chose the correct versions to get the required letter spacing.
When hot metal type was introduced, the letters making up each line of the text were assembled by machine, and it was impractical to manually select the right variant of each letter. Instead of a being fixed to a solid block of material, the letters were fixed to two “rails” at the top and bottom, and these were cut back at each side depending on the individual letter. So an A would have the top rail cut back on both the left and right sides, and similarly the bottom rail of the V. Therefore, the combination AV fitted together more closely than a combination like AB, where there was no cut-back on any corners of the letter B.
This system was a compromise, since it wasn’t possible to independently adjust the spacing for every possible pair of letters, but it works pretty well except for very large sizes of type, which were still set up by hand.
Computer type faces made the system more general by taking a step backwards: each letter was “fixed” to an imaginary rectangle, but there was a table of information showing the horizontal spacing adjustment for every pair of characters in the font, and all the adjustments were independent of each other.
That works fine for text, but it is not so good for music, because musical symbols are not typeset only in horizontal lines. Some software calculated the exact shape of each character, and used that set the spacing, but that approach is relatively slow, and works best with software that doesn’t support a graphics user interface for interactively editing the score (for example Lilypond or Score).
The SMUFL idea is to fit the characters into “boxes” which are rectangles with (optional) rectangular cut-outs at each corner. That restricted shape is much quicker for the computer to check than an arbitrary shape with curved sides, and is “near enough” to be useful in practice. It is more general that the hot-metal technology, because the vertical height of each cut out can be independent, and different on different characters.
AFAIK the SMUFL cut-out points are not part of the basic font format specification - they are additional metadata which is only useful with applications that understand SMUFL. I guess that in font editing software, you could draw “guide lines” that are not actually part of the character to visualize the cut-outs, but until somebody writes a SMUFL-compliant font editor, there would still be a manual step to store the metadata in the correct format.