I would challenge the above, but only in the sense that everything which makes it’s way into core is a conscious decision. The team has a direction for the product and are actively working every day to deliver on that. #pr-welcome means that while a change may be desirable, it’s not something they can currently direct effort toward internally. In that sense anything else is already either slotted into the schedule, or not suitable for core.
That doesn’t mean you shouldn’t suggest new features and capabilities, but always be prepared to be told that something you believe to be desirable doesn’t align with the goals for the default version of Discourse as seen in the standard install.
This is true for most open source software. Even the best written PR can be rejected for a multitude of reasons.