a few months ago i shared this thing by pretendo (a nintendo network revival wth a miiverse alternative) about how to dump and upload your 3ds/wii u network information so that they can keep the servers running after nintendo shuts them down. they've since updated it with a way to easily upload spotpass data from them too. recently, the spotpass archival project has also been formed.
this way you don't have to join the pretendo discord to upload spotpass data (but will still have to go there for network dumps). i uploaded mine to both places anyway. the site also mentions an even easier way to get a dump of spotpass data from the 3ds.