Would such a measure (or something similar) make any sense at all, does it limit the forum's freedom of expression and whether it can prevent the abuse of a newbie accounts?
I would say yes for thread locking but the one where the newbies can only create threads on a particular board would definitely limit someone's freedom. So thread locking is the best solution here since in this way you will really know if a newbie is interested in their own topic or not. 5 posts left unreplied is good but we can also make it better by seeing if the newbie member already made other posts from different topics, in this way you will know that the newbie is really not interested in their topic. I don't know if the latter can be done automatically in the forum so I suggest that thread locking can also be done with the user of "report to moderator" function so the mods can also lock it right away before it turns into a megathread.