Problem statement: When I share a folder with Editor permissions, all files uploaded by collaborators count toward the owner's account-level storage quota. There is currently no way to limit how much storage a single shared folder can consume. This creates two problems:

Unbounded risk – Collaborators (or with the upcoming public-upload folders, even anonymous link holders) can exhaust the owner's entire Drive quota, potentially blocking uploads, photo backups, or even unrelated workflows.
No project isolation. For professional use (e.g., project-based collaboration with external parties), there is no way to ring-fence storage per project without violating the Terms of Service by creating additional free accounts.
Proposed solution: Allow folder owners to optionally set a storage cap on individual shared folders (e.g., "max 10 GB"). When the cap is reached:

Uploads by collaborators are rejected with a clear message that the folder is full (the folder itself, not the entire account, remains fully readable).
The owner receives a notification so they can raise the cap, archive content, or clean up.
Why this fits Proton's philosophy: It extends the existing security-oriented sharing controls (password-protected links, expiring links, permission revocation) with a resource-control dimension. It is especially valuable now that public upload folders are on the roadmap. Those invite precisely the unbounded-quota scenario described above. Comparable capabilities exist in enterprise file-sharing products and are a recognized expectation for teams and professional users migrating from Google Workspace or Microsoft 365.

Scope for consideration:

Ideally available on all plans, at minimum on paid personal plans and Drive for Business.
An account-wide overview showing quota consumption per shared folder.