It's all client side -- someone (maybe me!) would spin up openkeychron from an old saved copy of the webpage and we'd be back in business in a day. Right now the keychron-hosted solution is fine -- no accounts or anything crazy -- so there's no impetus.

EDIT: turns out that my keychron, at least, is compatible with QMK/VIA and there are already open source alternatives [1] that do the job.

[1] https://usevia.app, https://remap-keys.app