Some Googling later, and I discover that both Resilio and DropBox use a similar extended attribute mapping schema and only whitelists a handful of MacOS specific attributes.
- DropBox: hardcoded list of Apple-only extended attributes and how they’re mapped onto other operating systems: What extended attributes does Dropbox support? | Dropbox Help
- Resilio: controlled by a per-sync folder whitelist file that by default only includes some Apple-specific extended attributes (can be extended by adding exact name matches). Stored in stub-files on file systems that don’t support it: https://help.resilio.com/hc/en-us/articles/204754729-Alt-Streams-and-Xattrs-in-Sync
- iCloud unsurprisingly also whitelists some Apple-specific extended attributes.