Only the latest Plumb release gets fixes. Each release is built on one Bitcoin Knots release; when Knots ships a security fix, Plumb follows with a new release on top of it.
Report Plumb security issues privately through GitHub: https://github.com/plumb-node/plumb/security/advisories/new (not for support).
If the problem is also present in the Bitcoin Knots release underneath, report
it to the Knots maintainer as well: luke+security+knots@dashjr.org, OpenPGP key
FAC0 98FE 8DF9 975F 9024 1881 3666 E2B1 782A 18E1.
Plumb releases are signed by Jason Sopko,
89F0 E41D 72CE 523F 4AA1 CDB6 92CD FFB7 C40C D1BA.