The one rule that covers 80% of timing decisions
Spring-blooming shrubs: prune RIGHT AFTER they finish flowering. They set next year's flower buds in summer on this year's wood. Prune them in winter and you cut off all the bud wood.
Summer-blooming shrubs: prune in late winter or early spring before new growth pushes. They bloom on new wood, so winter pruning forces strong new growth that flowers in summer.
That single distinction handles lilac (spring) vs butterfly bush (summer), hydrangea macrophylla (spring) vs paniculata (summer), forsythia (spring) vs spirea (depends on type).
Per Penn State Extension's pruning guide, the spring/summer bloom rule is the single most useful heuristic.