Notation
It is commonly thought that our standard notation originates solely from David Singmater. However, the truth is that every individual part was created by separate people.
Outer Turns
The community standard outer turn notation was created by David Singmaster.
The standard U, D, F, B, L, R notation appears in Notes on Rubik's 'Magic Cube'.
This notation is dated 1979.
David Singmaster states that he created his notation by combining John Conway's ideas with his own.
Conway, Benson, and Seal notation
Conway, Benson, and Seal used U, B, L, R, O, Y, which corresponds with U, D, L, R, F, B. As mentioned by Singmaster in the above interview, Conway had also been using a separate color based notation style previous to this one.
Apostrophe for Counter-Clockwise
The use of an apostrophe for counterclockwise turns originates from several individuals and Singmaster eventually started using that instead of -1.
Page 40 of Notes on Rubik's 'Magic Cube'.
Grip Notation.
Singmaster seems to be saying that people have requested a grip notation.
This is potentially the first publication to discuss the idea.
This quote appears in Notes on Rubik's 'Magic Cube'.
Slice Notation
M, E, and S slice notation comes from Frans Schiereck. (1981)
The book in which the notation appeared was De Hongaarse Kubus Voor Doordraaiers.
Rotations
It seems that Peter Jansen is who added x, y, and z to the standard notation. (~2001)
Wide Turns
The earliest publication of lowercase letters for wide turns that I found is from Dave Orser in October 2002.
The use of appending a "w" to the end of an outer layer turn for wide turns originated in the Japanese community.
Block Notation
James Straughan added block / group referencing to the standard notation. (Idea in 2010, developed in 2021)
Other notes
It's possible that others have had the idea of combining wide, inner, and outer turns to notate blocks, even if they weren't published anywhere. However, the 2010 proposal is the first known and the first complete development of the idea. It was also developed with confidence that it would be useful - and it has been. Many in the development side of the community have started using the notation versus describing blocks using sentences as was done previously.
Before 2010, there were many fewer methods than exist now. So there was less of a necessity and focus within the community on discussing specific block locations. So it makes sense that the idea wouldn't have been put out there. When the only quality methods were CFOP, Petrus, Roux, Nautilus, ZZ, and maybe a couple of others, it was easy to describe the steps using simple phrases such as "front right pair" or "right block" and have the methods not conflict with each other. But with more and more methods being developed and similar block shapes being built in different locations to what was done previously, the ability to describe these locations in a simple way has become useful.