There is no need to setup updating SpamAssassin manually as the package already includes a cronjob that runs sa-update daily (see /etc/cron.d/sa-update which calls /usr/share/spamassassin/sa-update.cron, which calls /usr/bin/sa-update and restarts the service when necessary).
According to the website
SARE is no longer active, so setting up a custom cronjob to also get updates from saupdates.openprotect.com is no longer useful:
IMPORTANT: Due to Ninjas being busy with lives, wives & hockey matches, SARE rules aren't being updated.
There is no need to run automated update tools as all they will produce is useless load on everybody's servers.
I've already e-mailed the people at OpenProtect about this and they said they'd add a notice to their page at http://saupdates.openprotect.com/
, but apparently that hasn't happened yet. They said they'd keep the service running to not break anything, although they said that indeed you won't be getting any updates from that channel anymore.
Anyway, we ran sa-update on January 1 when the update was released and all was fine.