[sciurius]

Jon Postel's robustness principle (a generally accepted design guideline for software) reads: "Be conservative in what you send, be liberal in what you accept".
Any program is completely free in accepting directives, abbreviations, whatever. Any program is completely free to accept "k" as an alternative way to specify the key of the song. But the ChordPro standard only defines "key". So programs are encouraged to use "key", but are free to accept "k" as well if they feel so. Do not think too easily that generated data is not meant for humans. Historic mistakes include PostScript, Forth, and XML.

That's exactly the point of a standard. Would be to state what "a:..." means.

Even if the standard did define a meaning, would you know it? At all times, without looking it up?
For example, I often use a id3 tagging tool that uses command line options -a for album and -A for artist. Or the other way around. Each time I have to lookup what means what.
On the other hand, the unabbreviated options are --artist and --album. Clear and unambiguous.
You lose nothing by generating unabbreviated keywords, but you may gain a lot.