Players who already have the game downloaded will not be forbidden from playing, but deleting the game may prevent those players from re-adding the game to their Steam libraries. The official news post makes it clear that players who wish to keep playing on American servers will not be locked out and that the client may still be downloaded directly; only the ability to do so from Steam will be affected.
TERA in Russia, you certainly can, as the game has a Russian publisher. The game is also available on Steam there. However, that’s caused a bit of an issue because the version of the game on Steam links up not to the local publisher but to the American version of the game published by En Masse Entertainment. The net result of this issue is that players in Russia and some parts of Europe will be unable to download the game from Steam starting on June 11th.
Source: Official Site; thanks to Zuji for the tip!