The marked_trail tag has relatively low usage worldwide and seems to largely overlap with the more widespread trailblazed.
A high proportion of marked_trail usage is in Norway. A common pattern seems to be marked_trail=T_beacon, trailblazed=yes. The ones I checked randomly were created quite recently.
Does this have an accepted meaning in Norway, and what does “T_beacon” mean in particular?
See also Is marked_trail deprecated?
Seems when I run it thru overpass, all relations connect it to the operator “Den Norske Turistforening” aka DNT.
DNT marks some their trails with the letter “T”
I can’t answer why marked_trail was used, long before my time, and I don’t map trails that often 
So I’ll leave that for someone else to answer.
But No:Map Features - OpenStreetMap Wiki does say:
Som sti, legg til
trailblazed=yes(tidl.marked_trail=*)
(Translates to:As a trail, add trailblazed=yes (formerly marked_trail=*))
Do they have one single account adding these or is it a myriad of people?
From reading the section of the Norwegian wiki you posted here, it seems to me this is supposed to be deprecated even in Norway? (trailblazing=T_beacon sounds like a good solution, if for some reason osmc:symbol is not enough)
I think this was discussed many years ago, back when the trailblazed tag was proposed, and that the conclusion was to move from marked_trail to trailblazed in Norway too.
