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…)
That’s a 20th level epic boon (so very powerful)
The power of a permanent truesight effect would be situational. If you measure ‘power’ by raw combat ability, then unless you’re fighting invisible creatures truesight has little effect. Even if this player is fighting invisible monsters, the rest of the party is still vulnerable. So it shouldn’t break combat balance. Outside combat, truesight would also be of little use if you don’t encounter illusions or shapechangers or ethereal creatures, although if you do encounter those things it solves those challenges trivially, which may be an issue for the DM trying to make interesting challenges. The range would also be a big factor. Truesight to a range of 10ft (for instance) requires the player to be standing very close to the item of interest and wouldn’t be as powerful as a range of 60ft, which would reveal an entire room.
But if we want to see how powerful the ability of permanent truesight is in general, we can look at where the game designers have made truesight available for player characters. You have already noticed truesight’s availability as a 6th level spell, which automatically puts it in a rather high echelon of power. But where would permanent truesight fit into the picture?
The answer is found in page 232 of the Dungeon Master’s Guide, under Epic Boons. Epic Boons are only available to player characters who are at level 20 and are made to extend the power of already-powerful 20th level characters. Of these epic boons, there is one which provides exactly the effect you are considering:
Boon of Truesight
You have truesight out to a range of 60 feet.
If the game designers thought that permanent truesight (to 60ft) is something worthy for only a 20th level character, you should be very cautious with giving out a similar ability to a lower level player character. Putting a much shorter range and/or limited uses per day might (might) help balance it for lower level characters, although it will still be very powerful in certain situations.