- Source attaching
Sure, I object to this complication as well, but less than the namespace reuse issues.
- Sending of multiple files in a single message (e.g. to send photo galleries), backwards compatible (legacy clients would see this as multiple messages). This is implemented at least in Kaidan, WIP in Dino.
Also supported not just by SIMS but even by OOB.
- Extension for proper E2EE (XEP-0448) when using SCE-alike encryption schemes (OX, OMEMO2), also backwards compatible (it's possible to send a file in a chat where some clients only do siacs-OMEMO such that OMEMO2+XEP-0448 clients see a proper SFS file transfer with metadata and legacy siacs-OMEMO+XEP-0454 clients get the "aesgcm" link, but both download and decrypt the same HTTP upload, so the file doesn't need to be uploaded twice). This is implemented at least in Kaidan.
This seems like it would also be supported by SIMS and OOB once we have SCE.