OpenWRT stopped ssmtp since 21.02. This results in an problem with memory, msmtp is not really an alternative to it.
I have the same problem like emilt in Ssmtp for 21.02, already installed openssl for uhttpd and not enough memory to install msmtp and its dependencies.
I think ssmtp is therefore a very important package, even if it is abandoned.
Edit: It seems that ssmtp is being developed further after all: [https://tracker.debian.org/pkg/ssmtp]
Has anyone an idea what to do ?
Since 03/2019 ssmtp has no active debian maintainer and has been marked as "orphan" (see https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=925056) for reference. The last changes are only non-maintainer uploads.
If you need the package so badly, become active yourself and take over the maintenance of the package.
general: maintainer: [Debian QA Group]
Since 03/2019 the version of ssmtp was developed from 2.64-8 to 2.64-10
You could just patch OpenSSL back in for msmtp.