I've stayed away from Showterm because it relies on a big rat's nest of Ruby deps. Looks like this one is written in Go, which is convenient.
Are these related?
Only in the sense that they are pretty much dead-on the same thing and Showterm has been around a lot longer and is in wider use.
When I see "previously" on HN, I tend to think "this was previously posted to HN." In this case, I thought the poster was indicating that asciinema.org was previously showterm.io - neither is the case.
asciinema was initially called ascii.io. It got renamed about 2 years ago.
No, shorterm wasn't around longer. I created asciinema in 2011.