In principle I support the idea of Omnis not being used to craft brand new (current year) tokens; that never felt like their purpose, and I was surprised when the program originally allowed it.
However, I have two major concerns about your post as written:
1. Permanently fixing the date threshold (at 2026, or anything else) seems short-sighted in the other direction. This year's new tokens will eventually become future years' impossible-to-find old tokens, and then we'll be back in the same boat we were in before. This is easily solved; if you limit their use to tokens which are out of print or at least X years
old, rather than a fixed date, then the Omni program can continue to do good things in the future.
2. The timing of this announcement is not good. Had it been made several months ago, I would have felt fine, even great, about changing the rules for 2027. But people are already buying (and awaiting the arrival of) 2027 tokens, and since some of those purchases were based on plans to craft 2027 legendaries using Omnis, it's kind of a feel-bad to yank the rug right now. To alleviate this, I would suggest grandfathering through the 2027 year, rather than 2026.
My suggestion:
Omni Cube
May be used in a Legendary recipe in place of one Relic token that is out of print (its Last Day to Transmute has passed) *or* is dated 2027 or earlier.
Omni Orb
May be used in a Legendary recipe in place of one Ultra Rare, or one Exalted, or one Rare, or one Enhanced, or one Uncommon token – that is out of print (its Last Day to Transmute has passed, and/or it can no longer be purchased from the True Dungeon store) *or* is dated 2027 or earlier.
NB: the "or" clauses will become redundant at the end of 2028, and can be removed at that time.
Alternative mostly-equivalent wording if you don't like "out of print":
... that is dated at least 2 years ago, *or* is dated 2027 or earlier.