Keywords are for convenience, not conceptual linking
Keywords (also known as “tags”) can be useful when notes are tied to an event, learning course, or other common thread, but they shouldn’t be used in place of inline links to other notes to communicate relationships between the concepts those notes represent. Relationships between notes should occur naturally within note content, since Note titles should express note content and Linking notes by title promotes conceptual cohesion.
Instead, keywords can be used to create siloed, loose associations in the note metadata that may enable filtering down to notes that were taken for a learning course or on a particular general subject matter. Keywords should be optional though, so as not to encourage using keywords as implicit links between notes.
A downside of including keywords in frontmatter is that the temptation to habitually add keyword(s) to notes will naturally lead to friction with the idea that notes can be created without keywords. If this happens, consciously creating keywordless notes is better than including keywords.