As a precaution, I left Thailand on January 4, and will be gone (except for 2 weeks in April) until July 1. If there still is no concrete information by then, I will spend another month abroad later in the year, so I am not a tax resident this year.
Letters from the tax office? Not likely. More likely but still a remote possibility, the Immigration office informs the visa renewal applicant that a tax return is a requirement for renewal - if the applicant is a tax resident. Since that requires coordination with the Revenue Department, and this is Thailand, this is an unlikely scenario.