Ugh, I'm in a similar situation ! None of the previous supported sites through the 3DS seem to work anymore....
The only reliable way I've seen to upload screenshots/QRs is by removing your SD card and putting it into your PC and accessing the images there. If you even have a PC that does that anymore LOL, mine doesn't (it does without SD card removal, which old 3DS can't do).
I tried a bunch of image sharing sites and nothing worked for uploading, but I did not try Twitter either. I also don't have it, so if it does work for you, please update !!