This is pretty huge news for Mastodon users: Once this feature is up and running comments to a post will not be limited to whatever is federated to your server through indirect means. The whole platform should feel a lot more vibrant after this, and it addresses what has been one of the main complaints about Mastodon.
It is summarized like this by @FediTips@social.growyourown.services:
Some good news about #Mastodon, there has been a very significant bit of code added which will automatically fetch all the replies in a thread. This means everyone will be able to see a thread's whole conversation no matter which server they are on! 🥳
The code has just been "merged" which means it should become available in the next update to Mastodon, hopefully within the next few months.
(Those who want the technical details can see the Pull Request at https://github.com/mastodon/mastodon/pull/32615)
While it is exciting, Mastodon CTO @renchap@oisaur.com points out that we maybe shouldn't expect too much too soon, although it is a big step in the right direction:
it is important to point out that this is very experimental, behind a feature flag, and there will be a need for more changes to get it in front of people.
Also we don’t recommend activating it yet, this may have significant performance implications or bugs.