Am I missing something? Agenda 4.0 on Mac. If I use ‘Search All’ and enter ‘a’ or ‘i’ without quotes, I get e.g. ‘No notes matching “a” in any of your projects.’ ‘Try using different search terms’. If I use any other letter, there are plenty of results, just as expected. Same behaviour on iPhone and, I expect, iPad. This is all independent of whether I have the ‘On the Agenda’ option turned on.
Agenda has stop words that it does not search for. Single letters are one such case. There would be so many hits, it would not be useful.
Agreed 99% of the time on single characters. The one case where it would be useful is if that single character is an emoji. Currently Agenda does not, to the best of my knowledge search on emoji. An emoji can be a rapid shortcut on IOS for conveying information. For example, I always use 
in place of ‘discuss with’ as it’s that much faster to enter. Being able to search emoji would be useful.
The need for this will drop a bit once reminders integration comes out as I can do the search on the reminders side. Also it is NOT as important in my mind as spotlight search which to me, trumps all, but it would be a nice to have.