I only know of two FOSS federated alternatives to reddit which is lemmy or kbin.
I think lemmy will scalebmore being written in rust were kbin is written in PHP.
And by scaling I am refering to high user count, high traffic, server resoures, server cost.
Without any knowledge on the details of both implementations: scalability has two dimensions, vertical where one server can handle more requests and horizontal where more servers can be added to serve requests. In recent years focus has been put more on the latter and for that the performance of a single node isn’t as important as for the vertical scalability.
To my understanding vertical scaling has limits which is the servers limitations as horizontal scaling if infinite since unlimited servers can be added
That’s true but total horizontal scaling is very hard for systems that need some kind of consistency properties
Will rust be able to scale horizontally in the future?
That isn’t so much a question of the language. Horizontal scalability is a property of the system you are building. You need to engineer the communication between the individual nodes, you need to make choices on the CAP tryingle (https://en.wikipedia.org/wiki/CAP_theorem) and more.
So rust is already horizontally scalable, it’s just hard to build horizontally, in any language.
(Sidenote, there is the language Erlang which seems to do a lot of this under the hood, but I have no experience with it so…)