限制的不是最小速率，而是最大通信速率。
0.25M也就是250000bps，换句话说，只要你的波特率小于250000就可以使用MAX483。
我只是比较了你选择的两个MAX483和MAX481芯片。
先来看Datasheet：

The MAX481/MAX483/MAX487 are put into shutdown by bringing RE high and DE low. If the inputs are in this state for less than 50ns, the parts are guaranteed not to enter shutdown. If the inputs are in this state for at least 600ns, the parts are guaranteed to have entered shutdown. See Low-Power Shutdown Mode section.

也就是说需要将RE拉高，将DE拉低，那么芯片就将自动进入低功耗模式。