Free and open source for selfhosting, paid when hosted by the company (e.g Nextcloud, gitea, cal.com)
Free and open source with basic features, paid for proprietary business addons (e.g Portmaster, Xpipe)
I think those approaches are fully compatible with the open source definition, but please correct me if I am wrong. (The examples I mentioned are just some of which I personally know and use, but of course they are many others)