How powerful would an always-on passive ability or magic item or whatever else that could grant truesight to a PC be? Let’s say 60 ft. to match the range of races with Darkvision.
For reference, here’s what truesight does (MM, p. 9):
A monster with truesight can, out to a specific range, see in normal and magical darkness, see invisible creatures and objects, automatically detect visual illusions and succeed on saving throws against them, and perceive the original form of a shapechanger or a creature that is transformed by magic. Furthermore, the monster can see into the Ethereal Plane within the same range.
On the one hand, the spell true seeing (PHB, p. 284) is a 6th level spell, which is pretty powerful, and only grants truesight for an hour per casting (although it does have an impressive 120 foot range):
This spell gives the willing creature you touch the ability to see things as they actually are. For the duration, the creature has truesight, notices secret doors hidden by magic, and can see into the Ethereal Plane, all out to a range of 120 feet.
On the other hand, a monodrone (MM. p, 224), which is only a CR 1/8 creature, has truesight, as do quite a few other “weak” monsters, which implies that it’s not that powerful.
So what should an always-on truesight ability be compared against to determine how powerful it is? It’s clearly superior to Darkvision, which some (most) races start off with. Is it on par with a legendary magic item, or is it less powerful than that? A feat, or is it more powerful than that? I can’t think of any other ways to grant a PC a passive always-on ability besides feats or magic items (outside something like a Blessing or something…)