By the time you're ready to summon Lutri, how often have you passed at least one turn with 3 mana up that you didn't need for anything else? Probably quite often, no?
The tax is sorcery speed. This totally kills Lutris ability to flash in and for you to hold up mana. It genuinely affects him disproportionately compared to the other 9 and very much makes him more vulnerable to targeted discard, because often you need to spend the 3 and sit him in your hand for a round. I think most of the other dorks will be taxed and played in the same turn once the game stalemates.
I actually think you can look over all 10 of these guys and figure out how the tax affects them proportionately to its play pattern. Ironically Lurris is probably the least affected by this because he was mostly a late game play / top deck war card to begin with.
- Yorion - greatly depowered, still probably playable in standard but changes it into a late game bomb vs a midrange one.
- Lurrus - Worse, but not by that much. Since he is low on the curve and all the stuff he's getting back is probably cheap to free I can imagine there being situations where he is actually better because you did not need the 3 mana but now he skirts Drannith magistrate
- Zirda - Weaker, but I assume when you play this you are hitting the I WIN button so probably not that much, maybe just delays it a turn. You can also still have 3 in the main to naturally draw and your whole strategy is based around making extra mana to pay for shit.
- Kaherra - Since the use case for this is creatureless deck backup wincon, I think the 3 mana is reasonable. Maybe in standard or whatever this takes a bigger hit because he was planned as an actual tribal lord
- Jegantha - Similar to Kaherra, though since he naturally costs more this tax hurts much more. My guess is it is unplayable now, and likely shops would rather run Umori if any of them.
- Keruga - is now more unplayable than it ever was. Trash now.
- Obosh - Maybe the biggest hit. Where as this saw play in Jund in some formats, there is no way this is worth 8 mana and all of your Liliana of the veils.
- Gyruda - His combo sorta does not work now as you used to be able to cast him off LED. Now you need 9 mana up front to trigger him into combo. Has maybe some main deck play in reanimator because it can pull huge nonsense in other formats, dead in vintage.
- Umori - The fringe shops decks that ran him to some effect probably want the sideboard slot back. The tax can be paid for but likely just not useable.
- Lutri - I think utterly unplayable now as mentioned before, as the deck building cost is way too high and the interaction with flash is telegraphed.