Thanks for the PR! If tail -r isn't available on all platforms where tac is missing I think we should fix it instead of changing the conditional. Is there an implementation of tac in Bash that will work on all platforms?

What about this for the tac implementation (taken from https://unix.stackexchange.com/a/9358/74959)?

cat -n | sort -nrk1 | cut -f2-

Can you test this out on your machine?

And, if what are you thoughts on changing the conditional to this?

if ! command -v tac &>/dev/null; then

You're right, it's better to make it fallback for everyone.
The solution you've proposed works on macOS Big Sur with default binaries.

/tmp $ echo A>a
/tmp $ echo B>b
/tmp $ echo C>c

/tmp $   tac() {
    /bin/cat -n "$@" | /usr/bin/sort -nrk1 | /usr/bin/cut -f2-
  }
/tmp $ tac a b c
C
B
A

Made the changes, anything else?

On a side note: caching for this plugin is heavily needed.

Thanks for updating this @arikai ! I was on vacation, sorry for the late merge!

Don't worry, just was trying to understand the circumstances :)
Hope you had a nice one!