It's been interesting reading all of this. We have had 3 Impreza Sedans since 2014 including the latest which is a 2019. We always use 87 octane gas and never heard any pinging. Subaru states the car is designed to run on 87 octane out from the factory so what might cause it ping? If it pings right when it was new that is an issue. It either means your fuel isn't really 87 or that something with timing is wrong.
If the engine starts to ping as time goes by this would indicate you are getting combustion build up in the cylinder which decreases the size of the chamber thereby increasing the compression ratio. The increased compression means you need higher octane fuel to prevent pre-ignition of the fuel before the spark fires. This will of course cost you more.
You can reduce or prevent combustion chamber build up by only using Top Tier rated gasoline. You can check online for which brand are actually Top Tier and if it doesn't hold the certification than it not. The fuel suppliers will have all sorts of fancy sounds names around their fuel like "Techron" but that is just a branding not a specification. Top Tier is a specific formulation of additives and a consortium of auto manufacturers and fuel suppliers created to improve the detergent characteristics of the gasoline. At any given brand you will find that all the fuel is or is not Top Tier, the octane rating has nothing to with it. TOP TIER - High-quality Fuel Performance Standard - TOP TIER™
You might wonder if this is all some kind of black magic hocus pocus to get you to pay more for gas. The American Automobile Associate wondered that and did some testing. They ran engines with and without Top Tier fuel and than took them apart. They found that after only 3000 miles the non-top tier engines were showing significant build up. The stuff works. Maybe its time for Subaru to become the second Japanese manufacturer to recognize the value of Top Tier fuel. There companies all recommend it: Audi, BMW, Ford, GM, Honda, Daimler Truck, Mercedes-Benz, Navistar, Stellantis,Toyota,VW
A study by AAA shows that Top Tier gasoline is worth the extra cost to consumers. Consumer Reports lists car brands that recommend Top Tier gasoline and gas stations where you can buy Top Tier gas.