It seems pretty silly for every Y ticket to automatically upgrade into F seat, even the last available one. If Y is going to book into F, why not just make it an F fare? If it used to work otherwise then this seems like a logical change. The cheapest F fare is usually going to be less than Y, anyway. Seems like the only people unhappy with this are going to be those who have someone else paying for their tickets who will pay any amount for Y but won't pay for F even if cheaper.