There is typically a pretty decent success rate when it comes to retrieving deleted phone information (texts, searches, photos). Specialized software makes it especially easy to process and analyze that stuff.
My vague impression is that the good olde days of easy retrieval might be in the past when a good number of criminals, office employees, and cheating husbands learned the hard way that hitting 'delete' did not truly delete anything.
Instead, the incriminating evidence was readily available for a period of time to inquiring wives, managers and others familiar with the software. Likewise, as you mentioned, it was often retrievable by police investigators for longer periods of time using common data recovery software.
But..... modern erase algorithms might be far permanent. Take for instance the case of the Apple phone and the San Bernardino terrorists. Breaking the entry password on the phone would have been easy for the FBI.
But... that would lead to an erasure of data that was evidently extremely hard to recover- even for the FBI. A series of court battles occurred before the FBI was helped by an unknown and very technically adept "friend" to salvage the data nd withdrew from Court.
Likewise, there has been discussion here as to whether or not conversations on
Snap Chat can be later salvaged and reviewed by anybody- even by agencies with state level technical departments or by the FBI with national level resources.
I wish I had more information. There might also be a difference between say a user hitting the delete function and somebody triggering a say, "deep erase" function.
