I remember facing this issue with FB Messenger. They used to have really good search options on it that allowed you to find old chat within the Messenger that you could screenshot and show as 'good' evidence. Then they limited back search to around 2015 and you could only scroll back very slowly. In the end I used an addon with Google Chrome to export all the chat history (from day 1) but the format is very basic. What I did was use this basic format to show initial chat, and then select random chat every 4-6 months to show it was ongoing. I did take proper screenshots of FB Messenger chat, scrolling back as far as the previous year to show valid chat in a FB application. It was a pain. So I would advise trying to get a few 'valid' screenshots, scrolling back as much as possible, and show the basic format chat from intervals as far back as possible.