That page on the Interrail site looks incorrect, has something major changed or is this an error?
According
to the Man in Seat 61, we have the RDG to thank for those supposed reservation requirements, and indeed it seems unlikely that Eurail would have made them up on their own.
In practice the whole thing is rather confusing, though - all non-English-language Interrail/Eurail pages still have the old information that no reservations are required apart from the sleepers, and indeed
I then discovered the site myinterrail.co.uk, as opposed to interrail.eu - it seems the former is operated by National Rail, which is a brand of the Rail Delivery Group (aka ATOC). This is the UK representation of Interrail.
National Rail's own (!) copy of the page
doesn't mention the reservations requirements, either. (Although at the same time this also means that their info
regarding validity on Heathrow Express is out of date, too... hm... sleeping dogs and so forth).
If you try the Interrail Journey Planner (which is advertised as the cool way to find out which services require reservations), that thankfully doesn't seem to show any domestic UK trains as actually being "Reservation
required", though. Plus as the Man in Seat 61 mentioned, Interrail passes alone are already exotic enough that it's unlikely that staff will actually know about those putative reservation requirements, so in practice it should be safe to ignore and/or feign ignorance about them if necessary. (And of course those TOCs that
aren't purely intercity, like Great Western or East Midlands Trains, will operate a number of services which aren't reservable in the first place).
It's still a rather worrying development, though, and I wonder what prompted it? Have there really been hordes of Interrail travellers descending on those trains, do they want to boost sales of Britrail passes, which don't carry such a similar restriction, is somebody dreaming of a reservations-compulsory world (see the recent Virgin Trains proposals) and starting with some small, easily overlooked target ...??