As a general rule, any project that mixes proprietary software in an "open source" repo (like the cal.com example) is a potential landmine. You have to be extra careful if you try to re-implement a proprietary feature since cloning the repo necessarily pulls in the proprietary bits. It's better to treat the projects as if they are proprietary,
As an open source project maintainer [1] with a set of proprietary add-ons, I totally understand the project maintainer desire to have a single repository with a single version, single build system, and a single place for issues.
I've considered doing the same thing, but it's all the legal complexity that stops me, doesn't seem worth it.