Ok, the solution is a plugin that does:
hooks.Filters.ENV_PATCHES.add_item(("mfe-lms-production-settings", "MFE_CONFIG['SHOW_REGISTRATION_LINKS'] = False"))
this is for MFE, while this
hooks.Filters.ENV_PATCHES.add_item(("common-env-features", "'SHOW_REGISTRATION_LINKS': false"))
is for the LMS/CMS.
No need to fork anything or patch any theme.