just a note that this is no longer the case - Cloudflare has patched this and R2 buckets are now compatible with S3. i have external backups to Cloudflare R2 and just did a successful full restore to another server. (it’s also free up to 10GB / month)
just need the bucket name (for s3 backup bucket), access key ID, secret access key and jurisdiction-specific endpoint from Cloudflare R2 object storage to put into the Discourse s3 settings or app.yml/web_only.yml etc.