Actual malicious code or malicious domains on one and the same IP could be a valid reason for avast! to create a general block of site or IP.
Outdated software and eventual free theme and plug-in vulnerabilities are never a reason to block a site, I fully agree with you there,
Never even suggested that could be a reason for the sites to be blocked. The blocking and unblocking and the reasons for that is a matter for avast team members. As a volunteer with some relevant knowledge I just try to shine some light on the fact that your website may come under attack and why that could be. That is all and there is no more to it.

With Sucuri’s however you should be in good hands.
However to avoid undetected exploitable software code it is always a good advice to fully update and patch.

polonus