I can barely recall this example sentence that I saw a long time ago:
Lei è la ragazza più bella che io conosca. (Not conosco)
It was used to demonstrate the wide use of subjunctive in Italian. The above sentence would be written in French as que je connais (indicative) or in Spanish as que conozco (also indicative).
I just saw this sentence on ThoughtCo as well:
Il razzismo era il peggior problema che ci fosse. (Again, not c'era)
Both sentences have a superlative expression in their main clauses, which drives me into thinking that the use of subjunctive in the subordinate clause is a special grammar rule in Italian.
Am I right? What are the details about this very usage of subjunctive in Italian?