I would even ask btree_gin and btree_gist to be moved to core.
btree_gist is shipping opclasses for built in types to be used in gist indexes. btree_* is confusing part in the name pretending there's some magic happening linking btree and gist.
gist is the most popular way to get geometric indexes, and these often need to be combined with some class identifier that's used in lookups together. CREATE INDEX on geom_table using gist (zooom_level, geom); fails for no reason without btree_gist - types are shipped in core,
gist itself is not an extension, but letting to use one core mechanism with another in an obvious way is for some reason split out.
