Hudson,
'high heels off' is a required call drummed in during regular QF evac training (tech crew do the same training incidently). It gets shouted out with monotonous regularity while you're waiting for the slide to inflate.
I'd find it difficult to believe (though not impossible, I suppose) that all of the door primaries omitted this. I'd find it much easier to believe that some pax just didn't hear it in the commotion, or heard it & ignored it. Time will tell.
Shoes other than high heels shouldn't affect the slides & there's no requirement to take them off.