It is very unfortunate that generic CalDAV and CardDAV are not supported by Odoo online. There are many people who do not use, and do not want to use, Google or MS for mail just to be able to integrate with Odoo fully.

This seems like an obvious way to be a support for open standards and low-cost solutions for all. It also appears that it used to be supported. I hope Odoo considers bringing back support for these.

I have the same question. I'm a solopreneur and would like to use Odoo Online Business for my business use. I hate GAFAM and would like to keep on refraining from using their services. Thus, I operate my own Cloud and Calendar using Nextcloud. IMHO a proper integration of Odoo Online service with Nextcloud should be standard feature. That would include the WEBDAV, CALDAV and CARDAV protocols. 

@Odoo what's your take on that? Providing a solution that excludes those to choose your service also because of its open source nature.