Hmm. The Vuelta stage Cavagna won solo a couple of years back ended with 850 metres at 7%, the final 600 metres of which were on cobbles (as per PRC), and Bennett beat Stybar, Gilbert and Valverde in the sprint for second. Démare has won a Paris-Nice stage ending with 1.9k at 5.4%, and in his 2020 superform could even win a Wallonie stage ahead of Gilbert and Van Avermaet that ended with 1.3k at 5.1%, after having survived a climb of 1.6k at 8.7% in the final 10k. Nokereberg is just under 400 metres at 5.3%, again on cobbles, so evidently well below the limit. The Tour stage where Kittel beat Coquard by millimetres was like 500 metres at 5%, so unless you're referring to that kind of pure sprinter I think the limit is a bit higher.