Actually the DA has two different disk space validations.
First is a set of dynamic deployment rules that the DA runs in which it checks things defined by release management or specific owners and for each version such rules can be different.
Second check is that from DA perspective the upgrade itself is going to succeed. For example - it has enough unpartitioned disk space to create a new partition, it has enough space under /var/log to extract the package etc.
Indeed if something is recommended only by documentation and not implemented on a rule - the DA will not enforce it
