A contact page now has Activity. Add a note there. Only the text is needed. Optionally mark it as a call, email, meeting or message. Notes show who wrote them and when, can be edited or deleted by anyone who can edit contacts, and say who edited them. Pin a note to keep it near the top of the page.
Activity also lists invoices sent, paid or voided for the contact, follow-ups marked done, and your own meetings with the person, matched by email address.
Logging a call, email, meeting or message sets Last contacted. You can also set it by hand. Set a follow-up adds a dated To-do for the contact. Contacts > Follow-ups lists what is overdue, due today and upcoming, and Contacts > Recently contacted lists who you spoke to last. The contacts list shows both dates, and searching contacts now matches the text of notes.
The old Notes field on a contact is now called About.