Is this in an absent minded way like a habit she's adopted while she's "busy" doing something or do you notice it with new stuff, as if she's assessing it? Little children are keen to touch, smell and taste as they explore the world. It can begin as a comfort mechanism.
I know you have to be super alert for any object that she might swallow or injure herself with but try not to make a big issue or you may find her doing this covertly. Unless she's in the garden tasting potentially poisonous things just calmly distract and occupy her.
One of my DCs used to lick mirrors and shiny surfaces, thankfully grew out of that!
My DS put everything in his mouth until turned 6. He will still 'regress' in this way if he is nervous, worried, upset or distracted - it's a brilliant visual cue for me to have some 1:1 time with him. The worst thing was clothing - all his long sleeves were always chewed and had to be re-hemmed, and looked dreadful.
But, he has grown out of it. We were consistent - at home we told him to take whatever it was out of his mouth, or we gently removed it (no fuss, except if it was something dangerous). As he got older, he needed a secret sign to get something out of his mouth when he was out - we did a nose tapping sign, because he didn't want to be embarrassed - he had identified for himself that this was 'baby' behaviour, and he wanted to break the habit himself.