EOL of iOS 12

You have already gotten the last update of iOS 12. It was a fluke that you got iOS 12.4.9, it was a security patch. The iPhone 6 Plus is a 6+ year old phone. I believe they stop support at 7 years., but there will not be any iOS updates.

No way to know. Apple does not really offer a hard date for end of life for their operating systems.

In general Apple can stop issuing updates for a specific version of iOS when the next version is released. However as is the case with iOS 12, they can continue releasing security updates and fixes for it if they think they need to.

As such, the better question to ask is, when does an iPhone 6 running iOS 12 stop doing the things you want it to do, and stop supporting the Apps you want to run. At that point, you can consider it EOL whether or not Apple is still putting out updates for it.

There is no hard EOL date to be given as Apple does not actually give one.

Welcome!

Very hard to say. For an official answer you need to ask Apple. We are end users lie you and can only comment based on observed history.

However, history says Apple supports the current system version and the two before it. Today that should be v12, v13, and v14. However, they have sent updates for much older versions when needed. Example: when a change in cellular service protocols would make older cellular iPads inoperable, Apple posted a patch for iOS 9 in July 2019, three years after the last update for that iOS version was pushed out.

Bottom line is I am not sure Apple has a set schedule. They have demonstrated that they will react to a need.