Is it save to delete old entries from the civirule_rule_log table
This table is used to prevent that cron triggers fire two times on the same day. So entries older than a day can be deleted.
The table is also a help debugging. For example, in the 'Manage CiviRules' form, they are used to show the latest contact a rule is triggered for. This information possible dissapears if you delete the older entries.

(But beware, in the Open Source world it is always possible that you have installed an extension that this table for important actions).