Well, I won't repeat my earlier rant, but the official integration guide says to look at the API Announcements category of this forum. And I think that's fine, but it takes the form of "announcements" of course, so one needs to piece together the announcements to find the upcoming timeline, as well as what pieces are relevant to what one is doing. So somebody coming in fresh (whether new to Let's Encrypt or just trying to solve a problem without having paid attention to the forums for a bit) doesn't really know what's coming unless they dig through the past announcements.
I think that one big challenge is also that there have been a lot of changes recently and upcoming, between the new E1/R3 intermediates, working on ECDSA issuance, and dealing with the DST Root expiration (with the plan changing in there at some point with the longer cross-sign). It's a lot to keep track of, and just coming in with "how do I ensure my system won't break" is tough to answer without looking through a bunch .