I don't know, because I don't know the Forgotten Realms.So what are the stats for clerics of Leira in 3rd, 4th or 5th edition?
But this is like asking what are the stats for Spirit Folk, or Hengeyokai, in 5e. And if I want to run a 5e Oriental Adventures game then I'll work it out, much as I worked out the stats for these races when I ran an Oriental Adventures Rolemaster game.
The complaint doesn't seem to have any substance beyond "They wrote a version of the Forgotten Realms, with 3E/4e/5e stats, and Leira and her (?) clerics were missing from it." Well, that's their prerogative. It's not forcing anyone to buy or play that material. Just adapt or make your own stuff, much as those who like the new material would have to adapt or make their own stuff if WotC had taken the pathway you prefer.
I've never had this experience, because when I run a setting I tend to rely on my verbal descriptions to convey details to the players. But when I think about the 4e FR Players Guide, for instance, I'm pretty sure its setting descriptions are mechanics-free. Likewise nearly all the descriptions in any of my Greyhawk books. So I would think that these could be handed over without much danger of inducing confusion. Just tell the player to disregard the chapter describing character options and focus on the stuff describing geography and history.From a different perspective, if I'm using the current Forgotten Realms, I can hand that edition's Forgotten Realms Adventures to anyone who wants to know what's going on. If I hand someone the 2E Forgotten Realms Adventures and tell them to take the rules from the 5E book, I've upped the confusion a lot. That may not be a big deal for some gamers, but it is for casual or new players.