@domo The feature was proposed as a Fediverse Enhancement Proposal: https://codeberg.org/fediverse/fep/src/branch/main/fep/e232/fep-e232.md.
Discussion happened on SocialHub: https://socialhub.activitypub.rocks/t/fep-e232-object-links/2722
FEP-e232 is implemented by a number of projects, but, as far as I know, Mastodon team plans to implement a different mechanism (which they don't discuss in public).