The issue for the organisation in the UK is that the Data Protection Act brings certain responsibilities to organisations regarding the protection of personal data. It also permits individuals to request data held about them.
As each congregation is a charity then each congregation has responsibilities under the DPA.
The trustees of the charity are the elders and, as such, they have numerous liabilities and responsibilities under charity law and other legislation, such as the DPA. Most elders have no real clue about what it really means for them personally.
For this reason and others, such as a reducing the risk of documentation being used against individual elders, the congregation or the organisation, then the org molly coddles the elders and prescribes, as usual, the detail about what should be recorded and how.
Audits of this documentation happen periodically. Any UK elder, especially if they have been the secretary or COBE/PO, should know this. I've not kept up with the detail of what is kept and how but the info has been discussed via letters and, more commonly, via elder's schools.
As far as I can tell this letter is not triggering some systematic cover-up and mass purge of documentation. It's simply making sure there are clear instructions to all elders to adhere to the existing policies. Rather than getting hung up on the letter, I think it is far more relevant to look behind this at what the instructions are for the recording and storage of information about JCs, child abuse and other congregation matters that may have a wider relevance. It's these policies and instructions that reveal more about the motivation of the WTS as to information gathering and storage.