When is the Magical dmg to towers coming to live?

You mean this?

#Turrets

##Damage to turrets on basic attacks from ability power increased and converted to magic damage.

Taking turrets is an important mechanic in the game, and mages often get left out of that game. Currently, champions who build more ability power than attack damage get some percentage of their ability power as additional magic damage whenever they attack a tower. We're upping that ratio, and also converting their whole attack to magic damage, not just the bonus damage.

• AP TO DAMAGE CONVERSION --- 50% ⇒ 60%

• ** NEW: THE WHOLE THING** --- If a basic attack uses ability power bonus to turrets, the whole attack will convert to magic damage, rather than only the ability power scaling portion.

Was part of patch 8.9, under "Turrets" (section below "Mages Changes"): https://na.leagueoflegends.com/en/news/game-updates/patch/patch-89-notes