You appear to have misinterpreted the meaning of this sentence:
An MQTT device trigger is a better option than a binary sensor for buttons, remote controls etc.
You believe it recommends one type of trigger over another.
It is not recommending the avoidance of “Legacy triggers”. It’s stating that to model a button or remote control, it’s preferable to use a Device Trigger than a binary_sensor (that’s a type of entity, not a type of trigger).
For future reference, Device Triggers are simply a relatively recent addition to the available set of automation triggers. There’s no such thing as “legacy triggers”. That adjective implies something is outdated and it simply doesn’t apply here.
Device Triggers were designed for use with the Automation Editor; they reference device_id which is stored in the device registry and neither accessible via a template or revealed anywhere in the UI. Long story short, they weren’t meant to be composed by hand.
Some of that may change in the near future because there’s a PR in the works to allow templates to access information in the device_registry, such as device_id. In other words, some of what you are requesting may come to pass but not because there was ever a recommendation to avoid using other kinds of triggers.
Having said that, it won’t change the fact that Device Triggers are verbose and tedious to compose by hand compared to State Trigger, Event Trigger, Numeric State Trigger, etc.