That feature was not universal; for example, MS DOS did not have it, as far as I remember.
Well, it did not, as it used the type ahead buffer the BIOS provided. By default that would be 16 bytes. It was quite useful to work at acceptable speed with diskette drives. Already these 16 bytes were quite useful and allowed to enter at least one command ahead, while the last was still processed. More so when extended by a TSR to 60 or 100 bytes. They showed up quite early. I could have never used a 4.77 MHz PC without.
The PC-BIOS was not the first to do so. Similar was offered by many BIOS for CPM, or other computers.
Furthermore, MS-DOS offered a read line function (Int 21h/0Ah, Buffered Input) to improve responsiveness for line input. This is not only handy for tool development, but guarantees shortest possible response between hitting a key, displaying it and executing edit commands (BS at least), independent of existence of buffers or buffer depth.
Just keep in mind how slow early computers were. If each character typed had to be handled all the way up to application level, usability for office application might not always be possible.
Which OS featured the type-ahead capability the earliest?
It's not really an OS feature, but usually located at the hard/software boundary. Many computers offered a type ahead. From the Apple II's 1 character deep buffer all the way to keyboard controllers like Intel's 8279 offering an 8 character buffer in hardware.
Likewise, back in the age of mainframe terminals, the whole character based handling was offloaded not only to I/O hardware but to external terminals/terminal controllers. A Block mode terminal, like the ubiquitous 3270 allowed complete local editing of not only a line but many thereof complete without OS (or application) intervention.
Similar for printing terminals like the 2740 family. Here a line is collected by I/O (not CPU) until finished by pressing Return or ATentioN. Only then the CPU is alerted and the line handed over for processing to whatever program (OS or an application using direct I/O) takes it from there.
After all, mainframes were block orientated by nature:
- Punch Card In
- Print Line Out
Block orientation is the very foundation of the high thruput these machines provided.
Long story short: Buffered keyboard input was there before unbuffered.
(Excluding experimental systems that is)