Triangle (Delta) chord symbol

[dflat]

Every time I see the triangle with a 7 I have a little pang of annoyance

Me too.

I am very curious as to when and how the 7 was added to the triangle ?
Was it started here at iRealb?

... (to answer your original question) no it was not us that started ^7 as an alternative to ^ by itself. However is has been convenient for us since we do not have room for 'maj'.

Personally I too learnt and prefer ^ without the 7 when meaning maj7. I wonder whether after some musicians started using ^9 that then the ^ was misinterpreted as meaning maj, so the 7 was added. Another point, ^7, ^9 and ^13 in the same chart make it clear.

We do offer both options so if you are annoyed, you can always change it (it annoys me too but I cannot be bothered changing them—sigh so I am used to it now).
Using ^9 (or ^13) might be specific for some reason, for instance the melody being the 9th or another instrument would be playing that note (big band) so the guitarist or pianist would be given this specific chord quality so they can voice it appropriately (and it gives them more voicings to choose from).