1990 Ford Probe ABS brakes

I think it is a big mistake to not get it fixed and would not call it safe. This is my personal opinion and say that from the abs system saving my life. Without it I clearly would have slid underneath a truck that probably would have decapitated me.

The anti-lock brake system (abs), allows you to have a much better chance of maintaining control should you go into a slide. If it detects a problem within the system, the light comes on and is no longer working.
The abs system will pulse the brakes in a rapid fashion that allows you to slow down when in a skid, it pulses faster than the emotional driver that is going into a skid that does not have a working abs system.

IT should be diagnosed.
There will be codes that will set in the system that are retrieved from the computer. OFten it turns out to be a wheel speed sensor that is the underlying cause.

My advice, is get it fixed before you need it.