Learn French: Il est interdit de toucher les fleurs. - It is forbidden to touch the flowers.

You definitely could! Saying On ne peut pas toucher les fleurs (We cannot touch the flowers) is perfectly normal in conversation. However, Il est interdit de is a set, formal phrase you will frequently see on signs meaning it is forbidden to do something.

Why does the sentence start with Il instead of C' (like in C'est)?

Il est interdit is an impersonal expression. When French uses 'It is' to state a formal, general rule rather than pointing out a specific object, it traditionally uses Il est instead of C'est.

Why is there a de before toucher?

The full expression is être interdit de (to be forbidden to). French often requires a small connecting word like de to glue an adjective (interdit) to the action verb that comes after it (toucher).

Why isn't the verb toucher conjugated?

Because it directly follows the preposition de. In French, when an action word follows a preposition, it always stays in its original dictionary form (the infinitive).