8MB is just the default, you can switch it back to 4GB if you want, but you won't have an easy time switching to 4.0 due to the 8MB limit imposed by FoundationDB.
It seems really odd to me that they are changing to an 8MB limit. "Document storage database" that can only hold 8MB per document. It can't hold full text output for a large documents... What am I missing here?
In CouchDB 4.0 the backend of CouchDB will be switching to FoundationDB, which will have a 8MB limit, so they're preemptively making the change. You can remove the limit now if you'd like.
FoundationDB needs is a consistent (as per CAP) database, to make that work in a distributed fashion, there are limits on distributed transactions in size and time. CouchDB needs to adopt those in order to move to FoundationDB.
The 3.x series will be supported for a long time for folks who can’t move up. Until then, as others have stated, you can up the default limit.