It sucks so much that there is no standard way of linking additional domains to your main one and inheriting the reputation.

Want to set up a new domain for whatever purposes (conference, new product, etc)? Be prepared to spend the first half a year fighting the various blacklists before people can actually reliably connect.

Would make so much sense if you could just have a .well-known/other-domains.txt (or something something DNS) with a list of domain names that should be considered just as trustworthy as your main domain.

It's not even about .online or other weird TLDs, it's just that the domain is new and therefore "not trustworthy". Even worse if you need to use your existing branding on the new domain - instantly flagged as a phishing site everywhere.