Is it a red flag if there's a new provider with 6 new TER reviews, that requires 50% deposit but all her reviews are from first time reviewers?  

50% isnt all that bad but when you get a lot of 1st reviews come up all at once its a big red flag,

As a lady who requires a 50% deposit, has no website, pro pics, socials or ads (yet)... BUT has a wee bit of history and a few reviews... RUN.  
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The biggest issue for me is that she is BOTH new AND all the reviewers are one-offs. That alone would kill it for me. Lack of website, socials, ads and the rest isn't necessarily a big deal on their own - lots of UTR girls operate this way, I did for 6 years. But one-off reviewers aren't going to be hooked into the kind of back channels where well established gents swap info on UTR ladies, so how would they have discovered her without any online presence?

I have required a 50% deposit to book, since 2010 & I am bonafide.

Run , Big red flag, i have been burned this way before.

….see what happens

Looks like she was taken down from TER and marked as a scam. Good news.