My 2 cents ...
As other pointed out, there is a large free shard uo community. These shards specialice in extaxtly one era. Some second age and rennaissance era shards reproduce the original era even close to 100% by going through the original patch note archives. We also know that subscription rates peaked during the ren era, thats why most of the successful free shards are based in that era.
Now, as UO was the pioneer of the mmoprgs, when the internet was the wild wild west, these free shards owners have been operating in a grey zone. By the time WOW came along, new rules were established and enforced (e.g the DMCA) and private WOW shards legally shut down by blizzard.
This has never happened with UO, either because of the legal loophole, as UO went live when the internet was legally a telephone line, or because of EAs incompetence.
For WOW, it makes perfect sense to release a nostalgia shard to cash in from retro players, but for UO, the hundred thousands of dollars returning players have spent to go back to their favorite era has been already leeched by the private shard owners.
Even if the original era-specific codes were available, few players will move from their private shards where they invested hundreds to thousands of dollars and countless hours just to start from zero here and pay a monthly subscription fee.
If EA hired an army of lawyers and shut down the grey markets, then there would be perhaps a chance for official era-specific shards, but that will not happen. They cannot even get legally rid of that daily russian RMT spammer who is clearly violating the TOS.
