No there is no map prefix as the gamemode uses no special map entites (It only needs spawnpoints really). I would just recommend using gm_ and then just put "Made for You Touched it Last" in the description, but if you wanted, I guess you could use the ytil_ prefix. However there are some guidelines I would recommend for this mode. This will be useful to everyone looking to make a map:

• Keep the map small
  Small hectic maps always make this more chaotic and by extension more fun. It should never take long to go to people on the other side of the map, so it won't cost too much on the bomb time. Think the golden number of players for this mode is 5, so size the map after that.
  
• Keep it open-air
  You'll not want an indoor map, or an open map that has big buildings surrounding you. It is mainly because you will want to be able to see the explosions on their full scale.
  
• Fill it with small things to run around
  Not big buildings or corridors, just small rubble/misc things (such as cars or small walls). This is because you will want to make it close-quater despite the map being open-air. Also no one likes doing that full power throw, only to miss a player and have to run across the map because no obstacles are there to bounce the ball back.
  
  Here's some things that aren't important, but often make it a bit better:
  
• Add a little bit of verticality
  Not too much, if it's too much then if you throw the ball after a person up there and it misses, it will fly way too far away. Just like a single floor or two higher. This is because it is always fun to be able to see all the action from above, especially the explosions
  
• Square/Circular maps
  Don't make the map too linear, you'll want to be able to run around, and running away from 'it' only to run to the other end of the map and have no other route to keep running is not all that good. Remember to keep it small too.
  
• Detail
  Everyone likes detailed maps.
  
  Here's some good examples of maps I think are great for this mode:
  https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=294357995
  https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=294363438
  https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=181162317

Ah, good to hear! I'll also keep those design reccomendations to heart. Thankfully, most of this fits my maps, which I plan to adapt to this mode, and use into the future.

RE the spawnpoint comment, how far does this spawn point recognition go? does it count info_player_start? info_player_deathmatch? info_player_rebel/combine? the counter-strike terrorist/counter-terrorist spawn entities?

Fairly sure it can use all of them. It just uses the regular player:Spawn() function which should just work like any other gamemode.

I like to use the map gm_stairs for this gamemode. The only problem is the spawnpoint.

Oh? why is that? do you know the entity that is used?

Honestly i got a pretty good thing at Parralax should be a good map. Close quarters,maze. It's awesome!

is there a map for this gamemode with an elevator?