Are Parties associated to Sale/Purchase removed from the navigation menu? (I pulled the latest tryton from docker, built it)
Yes, it was removed.
What is the new suggested way to differentiate between my suppliers and customers? And what if a party is both a supplier and a customer?
It seems like the current way to get customers is to use the sale report, grouping by customers.
I think it could be useful to have a relate from this view to Party, so that one can get a list of Parties who are customers.
Can I use the old Associated to Sale/Purchase logic, and add the old code to my custom module? Or Is it going to be problematic to do so
Yes, you can use the old logic in your custom module without issues. The primary concern is about naming consistency in relationships, not functionality. If that filter works for you, go ahead.
For context, the discussions emphasize linking parties only to validated sales or purchases, ensuring clarity about their status as customers or suppliers.