A month ago, references to S Mode for Windows 10 Home were spotted in a Bug Bash quest for Redstone 4, which is the update expected in the spring, possibly version 1803 that is now nearing RTM. It was rumored this would add to the already known SKUs for Windows 10 in S mode, with the existing versions being Pro, Education and Enterprise.
However, in a tweet shared earlier today, Joe Belfiore clarified that S mode will exist in all existing Windows 10 versions, and will guarantee a "low hassle / performance" version of Windows 10, mainly aimed at schools or businesses. This also means that Windows 10 S, launched last May, will not continue as a separate SKU.
As far as pricing goes, that still appears to be all over the place, earlier this month it was reported that users will be able to upgrade from S mode to Windows 10 Home for free, but a fee of $49 will be imposed on users wishing to go from S mode to Windows 10 Pro. This also points to the fact that Microsoft will be ending the free upgrade to Pro offer on March 31st, 2018, as stated back in September of 2017 when the offer was extended.
Source: Joe Belfiore (Twitter)
Update: We did a follow up post clarifying Microsoft's position on Windows 10 in S Mode and the cost to switch (which is free) here.