There is a different way of doing this that MIGHT be possible:
Legacy Kinships
With this you would be able to form a kinship, and immediately set a level limit of level 50, 60, 65, 75, 85, 95, 100, 105 or 115 - depending on where you want to be. A member of this kinship cannot level beyond the level set by the kinship, and a person that is of a higher level cannot become a member of this kinship. The level change can either be done by a leader, OR it can be done by SSG so that all kinships progress at the same time.
A kinship like this will exist on the current servers, using the current patch level, the current loot rules and open tapping rules, etc, etc.... this is a compromise that can't be changed, as others have said above it will be very cumbersome and almost impossible to maintain two different patch level servers, especially since part of your client is also updated and patched with each patch. So the entire world would be available (meaning a level 50 max kinship member CAN make their way to Isengard or Rohan or Gondor if they think they can survive the journey.) Quests like Vol II book 1 would be available at level 50 max, and you could have your basic legendary item while on level 50 max setting. These things cannot be avoided....
But kinships like this CAN better focus on the end-content of the kinship level setting, without every single character having to buy a stone of the tortoise, and without members accidentally leveling to 51 or so.... At 50, your experience will stop until the kinship gets its level increased to the next expansion level.
There are advantages to both ways of setting the new level: If in control of the kinship leader, the level can be upped when they are ready. If in control of SSG, say on a 6-month interval, you may have a better chance of finding quest and/or raid buddies, as it could be your kinship is too small....
In addition to a legacy kinship, you should also be able to put your single character in legacy mode, meaning you won't level beyond a certain level. Additionally, if a legacy kinship member or a legacy mode character STARTS a group to run group content, no one higher than the level set is able to join the group.... so no sneaky level 115 in the back that will half-solo the whole bloody raid.
A special channel called /legacy or /l for short can be created so that legacy kinships and/or legacy mode players can find other players to group with. The advantage of doing this over launching a separate server is that you can play your regular character on the same server, and monitor the /legacy channel in case someone needs your level 60 guard for a Moria raid. You can then switch to your legacy character and join the raid....
This COULD actually work as it would require relatively minor program changes, and it would co-exist on existing servers. All you need is a character slot or two so you can build a few legacy mode characters!