I agree with your suggestion, the welcome message should literally be pinned in every board on the forum so that when someone creates account and visit any board for the first time, they will see the welcome message, some new visitors does not view all boards at once that's why the message should be pinned in all board for visibility.
The number of times I have spent on beginners and help board, I don't see frequent questions about beginners basic suggestions you mentioned, the only time you see such questions are 1 in every 100 new accounts created. With that said, I believe beginners and help board has serve the purpose of welcome message to guide any newbie that doesn't understand the forum. The fact that Theymos had the intention one time but later abandoned it, then I don't think that update is coming back soon to implementation unless theymos had a change of mind.
There are couple of suggestions that were submitted last year, I'm not sure if there is anyone that was look into already. Maybe there is going to be any update I don't know but it's not going to be about welcome message. Theymos will not make an implementation if it's not that important or demanded by the community, don't bother if it's not security update about the forum or ways to improve users security(the last update like 2FA), sometimes privacy are considered like the "last seen" update.