... Not quite ; JustOff is a Ukrainian national and he's now (most probably) fighting against his country's invaders (not gonna say more, as being OT to these forums)...
Soon after the invasion took place, he shared maintainership of the github-wc-polyfill extension with a Chinese developer, whose GH username is SeaHOH (I don't know if I'm right but, based on my Chemistry background, I amicably refer to him as "seawater" ...)
SeaHOH is a very competent master of Javascript (with a slightly worse command of the English language compared to JustOff, but, OTOH, my Mandarin is non-existent ); I've reported several bugs myself over in the GitHub issue tracker and he's been very helpful/successful at addressing them ; TL;DR: All beta and stable releases after 1.2.14b2 are the product of SeaHOH's efforts!
The latest stable release, 1.2.16 , works wonders with latest Serpent v52.9.0 (2022-02-25) (32-bit); to install, visit the linked GH page, left click on the XPI link and accept the installation prompt from GitHub; else, if you want to first fetch the XPI to disk and then install, you'd have to modify manually the install.rdf file within the XPI (the maxVersion value); this has been explained repetitively inside these threads...
NB: The extension requires very recent UXP browsers (New Moon/Serpent), it does nothing in Firefox ESR 52.9.x and close to nothing in recent Serpent 55.0.0 (if force-installed there).
Loads and functions fine here:
(latest St52+gh-wc-pf-1.2.16)