SHort answer:
The old versions you have of SLP and WP are no longer supported. We do not maintain and cannot fix anything on those old versions . Only current versions are maintained. We are now on version 4.4.37 and about to release SLP 4.5 . There were major security fixes over the years by WP. Running old versions like that are opening you up to Malware, hacks and all kinds of issues…your error message is not from WP ..Sucuri is trying to tell you something.
Who told you not to update? If it was a theme author or someone else they must be using bad practices in WP and should really get updated.
I thought at first you meant not update to 4.5…but not update to even versions 4.4. of WP is dangerous and insecure.
Read these:
SUCURI found MALWARE in WP versions 2015
WP3Supercahce vulnerabilities 2013 article
WP 4.5 and why poor themes and bad jquery rules are just plain BAD
SLP author also see in that article for links to the WP support threads and other best practices