The current client date pre setup in rAthena is 2021-11-03, so I'd recommend the same.
Another famous client for Pre-renewal is 20180620, however if you use more robe costumes, it wont look good as 2021.
I believe this depend on the goal you are trying to set as a pre-renewal server, definitely there are some client bug fixes along the way. But most significant changes here is the feel of the client, the older the client, the lesser the features it will provide. so again it will lead to the goal of the server you are recreating.
Pre-renewal but with newer features 2021.
Pre-renewal in general 2018.
Totally pre-renewal, maybe you can go as far as 2015 client.
Just be cautious since if you use older client, there might be packet/compatibility issues as well, if you'll use the latest rAthena.