See http://www.squid-cache.org/Advisories/ At least the latest two seem to be unfixed in current Gentoo: SQUID-2018:5 (CVE-2018-19132), Oct 28, 2018 Fixed from 4.4 Denial of Service issue in in SNMP processing. SQUID-2018:4 (CVE-2018-19131), Oct 28, 2018 Fixed from 4.4 Cross-Site Scripting issue in TLS error processing.
I have added 4.x just now and masked for testing, still the bug would be irrelevant as we only have 3.x widely used yet, while 3.x and 4.x branches are quite different.