It should be clear right away why Umezawa's Jitte is banned in Modern: it does too much
While it must occupy your active slot to get its healing online, I cant think of many Grass decks in the TCG that wouldnt be better without a lumbering shield that not heals itself every turn, but also packs a punch
For one, land destruction is now red mana's domain, not white's, and current MtG cards are designed to not destroy too many lands too easily because that leads to unfun gameplay
Removing counters from this card or other artifacts will give you a ton of colorless mana to play with
The decks in Pauper are not well equipped to handle an onslaught of huge monsters in the developing stages of the game
Because this card has been around since 1994 and with each player having their own TCG lingo within their community, coupled with newcomers to the game tracking down the various pricing points for this card based on the numerous variations of listing is a challenge by itself