In the past I’ve dropped a few references  to the various tracks around Egmont National Park which have been ‘closed’ (in official terms) in relatively recent times. Reasons have generally ranged between erosion and lack of maintenance.
Here are a couple of recently-posted trip reports (not mine) on zoneblue.org , covering a couple of them.
Note that these are not (any longer) officially maintained tracks of the Department of Conservation. You really do need to take sensible precautions and have the appropriate skills if you wish to investigate. Especially the Pyramid Route. The first of these, in particular, is most definitely not a beginner experience. If you stumble on orange triangles on either of these routes it’ll be purely coincidental.
I just appreciate seeing information about them.
I should add that it’s also a slightly contentious point to talk about this. DOC in Taranaki has had issues  with people unofficially retaining old tracks.
The Pyramid Route : The high track above Pyramid Stream, between Holly Hut and Kahui Hut, has suffered substantial damage in recent years due to erosion around several of the waterways which it crosses. Eventually, DOC decided to stop officially maintaining it, although the route still appears on some older maps.
The Kiri Track : Contrary to the sketchy information shown on modern LINZ maps, it’s very possible to get to Pouakai Peak via Kiri Peak, from Carrington Road. One of these tracks is the Kiri Track, but there’s really a network of multiple tracks up that side thanks to maintenance from a combination of the New Plymouth Tramping Club  and some goat hunters. I assume it’s okay to note this association, given the NPTC’s President reported its track maintenance efforts to Radio NZ  a few months ago.
It’s certainly given me a few ideas of things to check out in the park, next time I have an opportunity.