Daytime naps become significantly more difficult after you get rid of dummy, because of the calming, soothing, settling effect the dummy has.
There's a thing called 'sleep pressure' - it means to what extent the child's body is put under pressure to go to sleep. It's natural that sleep pressure is greatest in the evening, at bedtime.
Sleep pressure is lower at lunchtime or very early morning. So while LO might still need to sleep at lunchtime (Or go back to sleep at 4.30am) they will find it more of a struggle to drop off because sleep pressure is lower.
It is these times when sleep pressure is low that the child (and you!) is likely to need the extra help a dummy gives them in order to go to sleep.
Baby probably would cope without dummy at bedtime at this age (after a couple of days anyway). But the effect of dropping the dummy is often that it is the trigger for dropping lunchtime naps. And can also result in early waking for some - although daytime naps would be the first and primary area to suffer.
That all explained, I'd suggest that 18m is too young to risk losing the daytime nap. For that reason I wouldn't drop the dummy completely in your position.
I would, however, ensure that dummy only ever lives inside the cot and is never, ever used outside of the cot.