I don't think there has been a widespread change since the concept of permitted routes was brought in as part of rail privatisation, though there has been some tightening up such as various journeys now being only "VIA YORK" as part of the LNER single tickets-only change which previously had much greater flexibility.  Easements do change, but generally on a temporary basis to take account of engineering work. 
It is I would say increasingly hard to spot useful alternatives to the obvious route, thanks to the National Rail Journey Planner being dumbed down to look pretty on a phone.  Even if you know an alternative route exists, it can be quite a battle to fool the algorithms to show the journey by using via points as just a few minutes extra on the journey time can render alternatives invisible.  In fairness most people would just be confused or irritated by being shown times they don't need, but perhaps a "show me everything however ludicrous" option should be offered.  Otherwise it can be a case of wading through the whole Routing Guide documentation, now held at https://www.nationalrail.co.uk/travel-information/routeing-guide/.

 
Well, services were improving to hourly or better and clockface in BR days, long before "any reasonable route" became "any permitted route" and the Routeing Guide came into existence.

The thread https://www.railforums.co.uk/threads/national-routeing-guide-update.99396/ starts in 2014, and the oldest I can find on the forum is from 2007: https://www.railforums.co.uk/threads/routeing-guide-easements.14305/

While these cover at-the-time changes, I don't know of any record of the cumulative effect of all the changes. I suspect that it would be difficult to research, especially as there are non-Guide changes that affect the choice of permitted routes. For example:

• Timetable changes that add or remove through services
• Some journeys not having an "any permitted" route option (only fares for specified routes)

 
KI'm not sure routing easements are removed are they? Do we have any examples?

 
Does anyone know of a source for how routing has evolved over the last... well, I'm not sure. Perhaps since the railway moved from a pattern of infrequent services over a rather more extensive infrastructure and started to aspire to, say, hourly clockface patterns of services on the network we use today.

Is guidance for passengers to the effect: 'If the National Rail site won't sell a ticket for it, then it's not a permitted route'?

An issue is that the National Rail site can give the impression that for some journeys that had an alternative that was rather longer than the arbitrary 'Up to 3 miles longer is a permitted route and anything else needs an explicit easement' rule, routing easements have somewhat contracted, which is not to the benefit of the intending traveller.

Mark