As I'd already spotted that the end of 1507 updates was imminent, this is the interesting bit for me:
Microsoft will no longer provide security updates for these editions after May 9, 2017. For more info, check out the Windows lifecycle fact sheet.
When I checked the lifecycle fact sheet last week, it talked about 1507 going out of mainstream support in 2020.
Now it's updated straight from 1507 to 1703 (missing out 1511 and 1607 completely for some reason) but the curious thing for me is that the date when mainstream support ends is still 2020.

Whatever happened to the idea of 'Windows 10 is the last version of Windows'?

If that were true, logically the support date for 1703 should have moved on a couple of years, because 1703 is a complete new release of an operating system so the clock should start again, and it should be in mainstream support until 2022.
So what's going on...?