Presumably you can select the relevant ID based on plan properties (cores, RAM, etc.) rather than hardcoding IDs. Though I completely agree that once a plan has a specific ID, as long as that plan is available the ID should be constant.
The plans themselves have changed many times before. They become obsolete over time. What if your plan ID becomes unavailble, or the specs change in a way your application can't handle? Don't shrug off your bad design decisions on someone else.
Deprecated plan ID requests should return two plan ids (new_price <= old_price and new_specs >= old_specs). Then a bit of retry logic means the code will still function sanely until the hard coded ID can be updated (or it may continue to function indefinitely).
Presumably you can select the relevant ID based on plan properties (cores, RAM, etc.) rather than hardcoding IDs. Though I completely agree that once a plan has a specific ID, as long as that plan is available the ID should be constant.