Using an older 5W charging brick on your iPhone 14 Pro Max will likely result in slower charging times, but it's not likely to have a significant negative impact on your phone's battery health, especially if you're not concerned about charging speed.
Modern smartphones, including the iPhone, are designed with advanced battery management systems that regulate charging to protect the battery from overcharging, overheating, and other potential issues. While faster charging can generate more heat and potentially stress the battery more, slower charging using a lower wattage charger is generally considered gentler on the battery.
However, it's important to note that using an underpowered charger might not provide enough power to charge your phone while you're using it heavily. If you're actively using your phone while charging with a 5W charger, the battery might not charge at all or might charge very slowly, and it might even drain the battery slightly if the power consumption of your activities exceeds the charging rate.
If your primary concern is battery health and you don't mind the slower charging times, using a 5W charger occasionally should not significantly impact your battery life negatively. Just be aware that using the phone heavily while charging might not result in much charge gain, and it's generally a good idea to use a charger with higher wattage when you need to charge your phone quickly or while using it actively.
If you find that your battery life is significantly decreasing over time, it's more likely due to the natural aging of the battery rather than the use of a lower wattage charger. All batteries degrade over time and usage, and this is a normal aspect of smartphone ownership.