Found all the Rebeccas of Mumsnet 
It is very dated. Doesn’t mean it’s not a nice name.
By the way, where I live, plenty of people pronounce it a bit like “Ribecca”, so not quite a ree sound at the start, but I understand what that poster is saying.
Mind you, I don’t know any Rebeccas who don’t go by Becky either.
I saw a post on here recently saying something like when the child introduces herself, all her peers will say “that’s my mums/aunties name” so it will seem more dated in that respect.
I have a very dated name myself. All my peers were Claire, Vicky, Becky, Jess, Nathan, Luke, Jamie... doesn’t mean you can’t use these names now but thinking on 30/40 years down the line when the person shares a name most common amongst pensioners, that could be worth taking into account