Check datapacks forum for old distros. I am hosting some on dropbox to which somebody linked there and there is rehost of it on some sharing site. They might NOT BE SAFE. Due to some weird coding in old distros there as a lot of false alerts, and while they are mostly well safe, there might be a virus. So proceed with caution and if you find something pm me, so I'll remove it from my dropbox.
And oldest I've seen was 7.1, I havent heard from lower.
WAIT
https://github.com/opentibia/server/commits/master?page=152
Yup, client lower than 6.5 was oldest otserv