I just want to say old.lemmy.world is fantastic. However, old can have negative connotations. Can we declare that OLD be an acronym for Optimal Lemmy Delivery?
Yes and no. old.lemmy.world is an mlmym client that is preconfigured to use the lemmy.world instance. The client and the API need to be compatible for them to work together smoothly. It is possible that, mlmym.org might update their client to match changes in the lemmy API, but lemmy.world could still be using an older version of the lemmy software. This could lead to compatibility issues between the client on mlmym.org and the API on lemmy.world, leading to loss of functionality or inability to connect at all.
However, since lemmy.world hosts their own instance of the mlmym client on old.lemmy.world, they have the option to continue using the older version of the client until they update their lemmy server software to work with the newer API version.
I apologize if this explanation is a bit long-winded and unclear; it's 1 AM here, so please feel free to ask if you need any further clarification.