Almost all examples I can find in the internet do not say "letter of recommendation", however there are a few which do.
It’s frustrating to have to look for a bunch of emails titled “Letter of Reference” to find the one about a particular student or post doc candidate. Alternatively, if I’m on an online platform, I want to make sure I’m reading the right letter.
So, as a matter of convenience for anyone handling the letter, I include a header
Letter of Recommendation for Anna Goodresearcher
or something similar as the subject of my letter. That way, it’s immediately obvious who the letter is for, making life easier for whoever is organizing or evaluating the applications. If it’s a postdoctoral reference or something else being sent directly to an individual by email, I try to include this in the email header as well.
Don't see why not, or why you should avoid it.
I really hate it when people stand on ceremony, and seem to be more worried about the format of something than the substance. No offense, but I don't think it matters, if anybody will read that letter of recommendation they'll be skimming through it.