In writing, a "beardline" is not a well-established term, but it could be interpreted in a couple of ways depending on context. If "beardline" is being used metaphorically or in a specific writing community, it could refer to a line of text that serves as a prominent point or highlight in a piece, much like a beard might be seen as a notable feature of a person's appearance.
However, it’s also possible you may be encountering jargon or colloquialism specific to a certain group of writers or types of writing (such as screenwriting or literary criticism). In some contexts, it could refer to the idea of a line that sets the tone or mood of a story, much like how a beard can change a person’s appearance.
If you meant a different context or a different term altogether, please provide more details for a more accurate explanation!