${var/word1/word2} is available in posix mode
The man page says:
/word1/word2 [...] This modifier cannot be used in the POSIXly-correct mode.
$ yash -o posixlycorrect -c 'var=word1; echo ${var/word1/word2}' word2
Fixed in r3579
Thanks!
The man page says:
In fact it can be: