Epic mobs get their true seeing so they can bypass blur, displacement and invisibility.
Invisibility is largely of use for those characters that haven't built for stealth, and are trying for a temporary boost.
True seeing doesn't directly boost spot, search or anything like that, the most it does for either of those is allow the seeing of secret doors up to DC30 which stops being a thing at about L15+ anyway, but it's not a boost to actual spot or search skills.
Essentially: the mobs get to bypass buffs, not skills. So this all very much depends on what you mean by 'stealth'. To me that's about the build - skills & gear.
For instance, my Rogue - 3rd life, few tomes, definitely only average geared - sneaks about in epics all the time so as to land assassinates and such, but she's not built for "stealth" beyond maxing out the relevant skills, I have not pumped dex beyond what's needed for TWF and if my gear has move silently and hide on it then that's entirely accidental rather than planned (and I'm pretty sure I'm not wearing any at the moment. There have been points whilst levelling where she's had that kind of gear because it also came with locks or disable or something on it.
And about the only time I find mobs having True Seeing to be an issue for sneaking about is when I've made a horrible mess of an encounter and need to get out of dodge - once mobs are locked on to you in DDO they pretty much are going to miraculously find you unless you can first break line of sight, then pop invisibility and then restealth (in that order) before they see you again.
Mobs with true seeing won't be fooled by the popping of invis but that is a theoretically optional step anyway if you can get far enough away from the mobs and properly break line of sight before restealthing. I've learned that if I get spotted, I need to UNstealth before I leg it, and then the REstealth will work more reliably once I have got a couple of corners between me and the mobs chasing me.
The bane of stealth is
- tremorsense - includes various mobs and some reapers
- arbitrary kill-all-mobs-before-the-door-unlocks choke points
And that second one certainly makes solo stealth play more or less non-viable, for me.
I don't know why the devs wanted rid of that playstyle, but they've successfully made it completley unnattractive especially once they stopped mobs being single-pullable via noise (e.g. a thrown dagger near to one of the mobs would bring that mobs and only that mob to investigate, that's not been the case for about a decade now, but it was BETTER I say. BETTER), so golf clap, I guess.
In a group (which is the only place I'd recommend playing a Rogue these days due to all the arbitrary magical chokepoints), if I get seen whilst scouting forward my response doesn't tend to be 'evade and hide' anyway, it tends to be 'leg it at full speed towards someone wearing armour (or the barbarian), pop diplomacy and then run in circles round them until their hate hoovers up the pack of mobs chasing me'.
By then assassinate's usually off cooldown so it's a quick stealth-and-murder job to help finish off the pack. Sometimes the rogue even survives....
Anyway. TL; DR - in what way does True Seeing get in the way of Stealth?