But they can deny you severance payments if you say anything bad about them. Not all companies do this, but enough that it makes the analogy to China or the USSR more appropriate
"The National Labor Relations Board (“NLRB” or the “Board”) held in a February 2023 decision, McLaren Macomb, 372 NLRB 58 (2023), that the “mere proffer” of a severance agreement which conditions receipt of benefits upon forfeiture of protected rights under the NLRA constitutes an unfair labor practice."