The Thai Labor protection act is clear, the maid, having worked for the employer for more than 10 years, is entitled to a severance payment of her current salary for 400 days. I does not matter if there is a written agreement or not, if she has worked for an employer, that constitutes an agreement. In a dispute, she must of course be able to prove that she has worked for the employer. And then of course we come to the question of tax payments, which should have been done by the employer.
https://www.ilo.org/dyn/natlex/docs/ELECTRONIC/49727/125954/F-1924487677/THA49727 Eng.pdf
(The original act was changed in 2017 and in 2019, google it).
Additionally, If the maid is over 60 years (since 2019, now officially the retirement age in Thailand) she can retire any time she wants and is entitled to the 400 days severance payment upon retirement.