Are you aware that Mastodon, Lemmy, Pixelfed, … are not encapsulated systems but just tools to participate in federated communication? There isn’t really such a thing as “a post on Lemmy” and “a post on Mastodon” - the systems are transparently working together to form the Fediverse where posts can be read, commented on, blocked, reshared, etc. independent of the platform you are using.

(Or in simpler terms: Lemmy communities can be posted to from masto and so on.)

Ecapsulating types of content on the system-level is therefore not really practical and not meant to happen in the Fediverse. However, content can be more or less forced into a small number of instances that other instances/user can then choose to federate with or to block them.