I'm no newsletter expert by any means, so, like idontknowyet, take my advice with a big grain of salt. But here's my anecdote, for whatever it's worth:
I don't use emojis or gimmick words. I just have the title of the newsletter followed by a number. That's it. I should add, though, that I give free books away (group promotions) in pretty much every issue of my newsletter, so I don't need to advertise "free" in the subject line. My biggest challenge has been in trying to turn "free" readers into "paid" readers. I'm still working on that.

I subscribe to Patty Jansen's Ebookaroo, and she has a kangaroo emoji in the subject line in addition to the words "free" and "cheap." However, she also has a message about whitelisting in every issue, so I'm guessing she's tripped some spam filters in the past.

Actually, now that I think about it, I think I had to whitelist Ebookaroo because it was being sent to my spam folder at one point.
I also subscribe to Jaxon Reed's newsletter, and his is simply titled "Jaxon Reed's Newsletter." No emojis or gimmick words. I read it every time, though, because the content is interesting. That's all that really matters, in my opinion: interesting content. No amount of emojis or gimmick words will save you if your newsletter's content sucks, and if your content is good, no emojis or gimmicks are necessary.
As I look at my inbox this morning, I see many promoters are doing the emoji thing now in their subject header: Bargain Booksy has an hour glass, 99cent books has a heart, Litring has a pic of books.
Advertisers tend to be trend followers. They do something because others are doing it, not necessarily because it works. It
may work, but the fact that it's trendy isn't
proof that it works. It's just proof that it's trendy. If you want to know if something works, or at least works for
you, then you'll have to experiment with it yourself and collect your own data.
Well, there you go. Hope that helps.
