Does existing iRules work at 12.0 ?

iRules is and remains a Tcl dialect language, so your rules from 11.6 should work fine under 12.0 without modification. There are no plans to eliminate, deprecate or replaces Tcl-dialect iRules. It will be supplemented by iRulesLX, but as I say, iRulesLX does not replace iRules. In 12.0, iRulesLX is Early Access, so at this point, there is no public facing documentation. See this solution article:

• SOL17293: Limited availability EA release of iRules LX