Early Leafs had batteries that didn't do well with heat, and didn't have active cooling, so I don't think many of those would have come close to 300,000 miles without major issues, which makes it hard to talk about long term life expectancy of stuff currently on the road. As far as publically available data, the best I know of is voltstats.net which lists metrics for gen 1 and gen 2 Chevy Volts, that owners have chosen to share with the site (the site pulls the data from On Star).
If we look at the EV miles driven of Gen 1 volts, based on vehicles still on the road, you've got a high water mark of something like this:
https://www.voltstats.net/Stats/Details/1502
That Volt has done over 150,000 EV miles in 7+ years.
If anyone else has better info, I'd love to see it. The main question is battery degradation, but very few EVs are going to have pushed past 200,000 miles yet, let alone 300,000 to give us data points on how easy it is for an EV to reach those totals miles.