The watchdog has written to complainants explaining it has reviewed all the letters that are still waiting to be sent out.
People who are due to receive them will be given 10 days’ notice.
Ms Yates said the apology was not good enough.
“It’s a bog standard cut and paste letter that has gone to everybody,” she said.
“I know there were other survivors that got letters which absolutely trashed their version of events.
“They had no right to do that. The complaint was about the conduct of officers. It’s completely irrelevant.”
She added the letters were “not an apology” and felt like they were “saying ‘We’re still going to call you a liar, just in a nicer way’.”
The IOPC said it expected to update bereaved families and complainants on its progress by the end of autumn.
It told complainants: “We understand that this can be upsetting, especially as you have waited a long time to receive the outcome, but this does not mean we do not believe your account.”
A date for the publication of its final report is yet to be confirmed.
In its apology letter, the IOPC said: “After we received the feedback, we immediately paused the process to consider how we could improve our approach and do everything possible to minimise any further distress.”
