Outside of spn.sh (and as an alternative to running wget --spider...), URLs can be saved to Wayback Machine like this:
>$ cat urls.txt | xargs -d "\n" sh -c 'for args do echo $args; curl -s -m 60 -X POST --data-urlencode url="$args" -d "if_not_archived_within=157680000" -H "Accept: application/json" -H "Authorization: LOW api:key" https://web.archive.org/save/; echo; sleep 50; done' _
IA/WBM API key at https://archive.org/account/s3.php - save IA API key to ~/iakey.txt then replace "api:key" with this:
>$(head -n1 ~/iakey.txt):$(tail -n1 ~/iakey.txt)
More info: https://archive.org/developers/ias3.html

What I mean by "spn.sh":
https://github.com/overcast07/wayback-machine-spn-scripts/
>./spn.sh -a "api:key"

However, when running the above "cat urls.txt..." today got this error which I think is about remote memory usage:
>{"exception":"Command # 1 (LLEN spn2-api) of pipeline caused error: command not allowed when used memory > 'maxmemory'.","message":"Cannot start capture","status":"error","status_ext":"error:celery"}
- Celery? I don't even like celery.

More fails:
. wget --spider = HTTP 520 UNKNOWN on https://web.archive.org/save/_embed/https://... requests recently.
. https://web.archive.org/save/https://vignette.wikia.nocookie.net/mlp/images/1/14/Applejack_%22Apples%2C_carrots%2C_celery_stalks%22_S1E11.png/revision/latest?cb=20140606032105 = "Sorry \ Cannot start capture"