User blog comment:KillRoy231/Will it end at Season 14/300 episodes?/@comment-2605:A000:122A:408A:E1E1:AEE0:C68E:80D5-20180315085530

There's an article in Entertainment Weekly in which Jared is quoted as saying once they reach 300 "I think Ackles and I will think it may be time to say good-bye." In the same article Jensen is quoted as saying "In a marathon, I keep my sights on that next mile, whatever that may be.  And  300's a good number to work for." http://ew.com/article/2016/09/06/supernatural-jensen-ackles-jared-padalecki-show-ending/  This is Sept. 6, 2016. Then Sept 21, 2016 Variety publishes this:

Both Ackles and Padalecki insist they’re not putting an expiration date on the series. “Jared and I talked and we’re going to just keep going,” Ackles says. “He and I, we talk about getting to episode 300 and that’s just another milestone… and then when we get there, we’ll keep going and see what the next milestone is. One of the many reasons we’ve managed to get to where we are is, I don’t think we really set parameters or limits or any obstacles. It’s just like, ‘Let’s keep our heads down and let’s just do the best work that we can and hopefully, people will continue to watch and we can keep telling these stories.” http://variety.com/2016/tv/news/supernatural-season-12-ending-the-wb-cw-1201866035/

I know these sources are a year old, but this is when the 300 rumor started as near as I can recall. Jensen also denied the 300 episode ending in Oct and Nov of 2016. But maybe now they really do want to end it. I think that the more people assume that 300 is the end, the more likely it is to happen.

I did see Jared respond to a fan's question during a convention (don't ask me which one--I saw it on youtube. but it was sometime 2017-18 tv season) that he couldn't wait to start acting and now he can't wait to stop. I haven't seen or read anything that officially declares 300 as the end. Maybe someone else knows of it.