just to be sure...
It is ok to delete features/rows in a non versioned feature class/table using SQL DELETE, right ?
Yes, of course. The only exception was when using SDEBINARY geometry storage, when a layer was in load_only I/O mode, where the triggers were disabled, so that a DELETE would not remove the Sn and Fn rows. Most modern databases use ST_Geometry or native SQL storage, so this doesn't even apply.
In addition to what Vince said, make sure it's not participating in a relationship class.
Retrieving data ...