Taylor Blau [off-list ref] writes:

When performing a garbage collection operation on a repository with
unreachable objects, Git makes its decision on what to do with those
object(s) bed on how recent the objects are or not. Generally speaking,

s/bed/based/ ?

+gc.recentObjectsHook::
+	When considering whether or not to remove an object (either when
+	generating a cruft pack or storing unreachable objects as
+	loose), use the shell to execute the specified command(s).
+	Interpret their output as object IDs which Git will consider as
+	"recent", regardless of their age. By treating their mtimes as
+	"now", any objects (and their descendants) mentioned in the
+	output will be kept regardless of their true age.

Thanks! I think this version will be clear to prospective users.

+static int run_one_gc_recent_objects_hook(struct oidset *set,
+					    const char *args)
+{
+	struct child_process cmd = CHILD_PROCESS_INIT;
+	struct strbuf buf = STRBUF_INIT;
+	FILE *out;
+	int ret = 0;
+
+	cmd.use_shell = 1;
+	cmd.out = -1;
+
+	strvec_push(&cmd.args, args);
+
+	if (start_command(&cmd))
+		return -1;
+
+	out = xfdopen(cmd.out, "r");
+	while (strbuf_getline(&buf, out) != EOF) {
+		struct object_id oid;
+		const char *rest;
+
+		if (parse_oid_hex(buf.buf, &oid, &rest) || *rest) {
+			ret = error(_("invalid extra cruft tip: '%s'"), buf.buf);

To be consistent with the other error message, perhaps s/extra cruft
tip/additional recent object/?

+static void load_gc_recent_objects(struct recent_data *data)
+{
+	const struct string_list *programs;
+	int ret = 0;
+	size_t i;
+
+	data->extra_recent_oids_loaded = 1;
+
+	if (git_config_get_string_multi("gc.recentobjectshook", &programs))
+		return;
+
+	for (i = 0; i < programs->nr; i++) {
+		ret = run_one_gc_recent_objects_hook(&data->extra_recent_oids,
+						       programs->items[i].string);
+		if (ret)
+			die(_("unable to enumerate additional recent objects"));

This error message to be exact.

+test_expect_success 'gc.recentObjectsHook' '
+	git init repo &&
+	test_when_finished "rm -fr repo" &&
+	(
+		cd repo &&
+
+		# Create a handful of objects.
+		#
+		#   - one reachable commit, "base", designated for the reachable
+		#     pack
+		#   - one unreachable commit, "cruft.discard", which is marked
+		#     for deletion
+		#   - one unreachable commit, "cruft.old", which would be marked
+		#     for deletion, but is rescued as an extra cruft tip

Perhaps we intended to drop "cruft tips" in the test comments too? e.g.
here and..

+test_expect_success 'multi-valued gc.recentObjectsHook' '
+	git init repo &&
+	test_when_finished "rm -fr repo" &&
+	(
+		cd repo &&
+
+		test_commit base &&
+		git branch -M main &&
+
+		git checkout --orphan cruft.a &&
+		git rm -fr . &&
+		test_commit --no-tag cruft.a &&
+		cruft_a="$(git rev-parse HEAD)" &&
+
+		git checkout --orphan cruft.b &&
+		git rm -fr . &&
+		test_commit --no-tag cruft.b &&
+		cruft_b="$(git rev-parse HEAD)" &&
+
+		git checkout main &&
+		git branch -D cruft.a cruft.b &&
+		git reflog expire --all --expire=all &&
+
+		echo "echo $cruft_a" | write_script extra-tips.a &&
+		echo "echo $cruft_b" | write_script extra-tips.b &&
+		echo "false" | write_script extra-tips.c &&
+
+		git rev-list --objects --no-object-names $cruft_a $cruft_b \
+			>cruft.raw &&
+		sort cruft.raw >cruft.expect &&
+
+		# ensure that each extra cruft tip is saved by its

here.

Aside from those trivial points, everything looks good.