The Rock Band team announced via Twitter on September 22nd that the export process to bring songs over from Rock Band 3 into Rock Band 4 will be expiring on December 1st. The tweet lays out a little more detail:
On December 1st, the Rock Band 3 Export is expiring. We know it’s a bummer, but you still have time to purchase an export key. If you have owned and played Rock Band 3 on the same console family or profile and want to export a bunch of rad songs to Rock Band 4 – act soon!@RockBand tweet
This is the latest (and presumably the last) time that Harmonix will cut off the export ability for Rock Band 4. Over the past few years the export purchases for the original Rock Band, Rock Band 2, LEGO Rock Band, Green Day: Rock Band, and Rock Band Blitz have all come and gone. If you didn’t buy the export key at the time, your chance has passed.
If you’ve been holding off on bringing your Rock Band 3 songs over, now’s the time to finally get it done! I’ve added the deadline date to the Watch List calendar. Thanks to @Mono_2024 for pointing out the news to the site.