Question: why vault-ruby gem is treated as old client?

As per these comments old clients use “sys/renew” while new ones use “sys/lease/renew”.

Why the vault-ruby is treated as an old client (trial and error helped me figure out that I need the old-client policy for lease renewal) ?

Because the authors of vault-ruby haven’t updated it, I guess.

You are right:

They use the old path. Yet, the documentation doesn’t mention about the old API and I was wondering why the policy doesn’t work