How is the PMM team structured by product line, segment, function or by objective (ie. revenue retention, product engagement, etc)?

I think that depends on your company goals. At HoneyBook we've identified a specific segment that has the best product-market fit, so we're solely focusing on that segment. Therefore it makes more sense for us to split the work by feature (product line) and by user lifecycle stage. But I think that what's most important is to understand what are your company's priorities and organize your team in a way that supports those priorities. If you work for a startup, these may change year over year, so you have to stay flexible.

I've seen organizational structures of all types over the last 10 years and have grown to appreciate a function with clear roles and responsibilities. The most common configuration (and the one I've had the most success with) is where I can have a leader for Core Solutions (your revenue generating products and SKUs), a leader looking after the Portfolio (someone who thinks about how to bring the portfolio to different audiences), a leader looking after Pricing and Packaging (who can work with your finance, product, and revenue teams to maximize profitability), a leader looking after Compete and Marketing Intelligence (win/loss analysis, compete, and dispositioning), a leader looking after Customer Marketing (advocacy, references, advisory - as well as partnering with Account Management to drive upsell/cross-sell with a remit for revenue retention) and a leader who can help manage projects, programs, and operations (i.e. Launch, Reporting, Analytics, and cross-functional dynamics). If the organization doesn't have Enablement, it will most likely be your Core and Portfolio leaders working with the field directly.