Confused About Special Section D

Hey all! Hope you're all doing well, congrats on getting through the holidays for those who struggle :)

I (FtM) have been going through the whole passport and work documents change ahead of applying for work, and I am confused about the whole Special Section D thing for HMRC. I changed my name on my national insurance number a few years ago, but I don't recall requesting my files to be hidden as I wasn't aware it was something I could do.

I've just read over the gov's website on it (screenshot below) and I'm confused as to how I'm supposed to request a Special Section D so that my files can be locked, as I want to remain stealth at work. The gov website says that you cannot confirm your legal gender change over the phone, which I assume refers to the whole GRC process? I am yet to apply for a GRC so is it possible to have my files locked? The gov website says you can only tell them about your name change only if you have not changed gender legally ://

If I were to request it, am I able to do so via a letter? Phone calls stress me out so much at the moment, and I'd prefer to have written evidence that I have made this request.

This is the page I'm looking at btw https://www.gov.uk/tell-hmrc-change-of-details/gender-change

If anyone could be of any help, I'd greatly appreciate it :)