In my area there was a lot of mining over the past centuries, but completely closed down since long. Scattered all over the area you find lots of relics from these activities, most of them easy to tag.
Recently I ran into the remains of an old ore cable-car line transporting the ore from various mines to the smelting plant. The line has gone since long, but alongside one can still find the concrete foundations of the former steel pylons (which have been removed completely). Most of these foundations are located in forest areas today, some on meadows or farmland. One set consists of 4 - 6 massive concrete blocks, some 1x1 m base and up to 2 m high, surely visible OTG for the next hundred years and therefore well worth to be tagged. Another type are the foundations of the former mine shafts which are even bigger.
There are some 30 man_made=foundation tags acc. to taginfo so far which makes me wonder if there might be another tag for those - there must be thousands of such objects worldwide imho.
it is entirely possible that there is no standard tag - I encountered many similar questions before
man_made=foundation makes sense, and makes clear that only foundations are there (for me abandoned:aerialway=pylon would be rather something where pylon structure is still there)
I have used several lifecycle prefixes already but in the acutal case none of them would really fit according to my understanding. The former pylons are not abandonded, demolished or ruins. One could say this are razed:aerialway=pylon but the object to be tagged is not the (non-existing) pylon but the still existing (and even intact) foundations.
The best fit in my opinion is man_made=foundation as this is what is visible OTG - I was just wondering why this tag is not in use far more often.
OK probably there will be some other input so I will leave this topic open for some time but if no better idea comes up I will use man_made=foundation + a short description.