Hello Tony, this is largely undocumented, yes you can use 7+. (I think this happens with using "maj" and "dim" also instead of their symbols). I seem to remember we originally did not allow these equivalent chords but we found some users putting them in then wondering why they did not work, so we added in a few, but yes, it still shows as not recognised, although plays.
As you know there are a number of ways (perhaps too many ways) for chord symbols to be written. We may change this in the future so they show as being recognised. As it is, it encourages consistency with iReal Pro charts, and beginners do not have multiple symbols meaning the same thing.
You will understand why "dim" (and "maj") is not used commonly, as they take up too much space.

We will fix Crosscurrent, thank you.