Mastodon's latest version has the ability to fetch missing replies from a conversation (under an experimental flag, not enabled by default).
This is a big deal, especially for smaller instances. Mastodon joins other platforms - of which I know of: Streams, GoToSocial, IceShrimp.NET and Bonfire - that pull back replies on request.
The hard work on the backend is done, and now there needs to be a discussion about how this interaction works as part of the user experience.