-i is definitely not specificed by POSIX, but it supported on all those platforms with some small differences, for instance on OSX the backup extension (-i.bak) is not optional.

Amazingly, there is no portable way to use -i that works on both GNU and BSD sed implementations. Which means if you’re writing a portable script, you can’t use -i at all.

(Would love to be proved wrong on this)

According to Stack Overflow [1], this works:

    sed -i.bak 's/foo/bar/' filename

[1]: https://stackoverflow.com/a/22084103/3266847

That works when specifying a backup extension, but not if you don’t want to create a backup file.

    sed -i ‘’ ...

works on BSD sed but not GNU. Meanwhile:

    sed -i’’ ...
    sed -i ...

both work on GNU but not BSD.

Well, yes, you can't use it without creating a backup file, but it uses "-i" and is portable.