When a roadmap is developed using agile or continuous deployment methodologies, sharing a long-term roadmap (say for one to two years) usually means that you share a general plan and timing, but one that shows your plan without set-in-stone dates. This enables you as a product owner to achieve alignment and buy-in for a strategy, while still providing flexibility down the road to shift an actual delivery by a few sprints or weeks.
You can share a detailed long-term roadmap without firm date commitments using an External date on a release. Even if you may still be tracking to a particular release date for a new customer experience, this date can stay internal and for your team's use only. The external release date can have the precision you are comfortable with sharing, based on the current date and how far out in the plan you are communicating. External dates can align to the actual date, or be generalized to a month, quarter, half year, or full year precision.
Below is an example of a plan by quarter, showing some release and feature detail, but with external dates that have less precision the further out the plan is shared.
Sharing a plan with external dates can be formatted in any way you wish in order to create the right kind of report for the stakeholders you are considering. Below is an example of a pivot table showing a long-term plan by quarter.
A roadmap with external dates still represents a commitment, but with a precision that is less defined as you go further out in time. By sharing a roadmap internally, collaborators see a baseline and can more easily pinpoint suggestions — ultimately creating a stronger plan. With external stakeholders, it establishes trust that they are part of the process, and builds excitement for what's coming up — but you've only shared the information and precision on the plan that you're comfortable with at a point in time.