Some discussion here:
But the short answer is there isn't currently a good way to expand all branches at once, just the next level (or next N levels). So you could expand, say 10 levels deep and cover most cases, but not all.
Of course, Flat View (Grouped) is still an option if you want to see everything at once; you just can't then collapse individual branches (other than by exiting Flat VIew and going back to normal, collapsing everything). But if you want to expand everything and keep it expanded, that's what Flat View (Grouped) does (same as in Opus 12).