[OFFICIAL] Last Hit Indicators 🎯 will be enabled Normal Draft & Ranked Summoner's Rift games on Patch 26.13:
"We think it's better to focus on skill expression in other parts of the game—rather than understanding exactly when you can last-hit a minion—and are also pretty confident that these don't degrade skill expression in lane. You still need to be mindful of positioning, harassing your opponent, considering where both junglers are, understanding power spikes, and more."
Via Patch 26.13 Notes