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.
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…)