Hmm! Interesting. Just not showing up under Extensions in v20. I have also tested PL back a far as v16 and with the last of the Sony Vegas releases which was v13 and PL v 4.1.11, is working fine in all those versions.
Upon investigation, there was no C/ProgramData/Vegas Pro/Application Extensions Folder in v20. I copied that Extensions folder from v16 to v20 and now PL v4.1.11 shows up under Extensions. Go figure??