In 2015 I would assume you used a letter from your embassy to prove you income to apply for the extension.
In December of 2018 immigration issued a order allowing the income option to proven by transfer in a Thai bank for 12 months. It was done since at least 3 embassies (UK, USA and Australia) stopped issuing a proof of income document.
If can still get that proof from your embassy you can still use it.