Would love to see this, e.g. just being able to pin interesting/failed puzzles for review.
Saving Puzzles

After solving a puzzle, you could click on the analysis button for the puzzle, and then click on the "add to library" button.
Due to a current bug, the "add to library" button is disabled.
For now, you can click on the download button for the puzzle, copy the puzzle PGN, and then save the PGN to your library.
As a software developer, implementing a feature that allows users to save puzzles for future study could potentially increase user engagement and retention. It could also potentially lead to increased revenue if the feature is offered as a paid add-on or part of a premium subscription package.
From the perspective of a customer, having the ability to save puzzles for future study can be very beneficial for improving chess skills. It allows the user to focus on a specific type of puzzle or position, and track their progress over time. This can be a useful tool for both casual players looking to improve their chess skills, as well as more competitive players preparing for tournaments. Additionally, being able to save puzzles can be convenient for users who may not have the time to solve all of the puzzles they come across in a single sitting, as they can come back to them at a later date.