Both, depending on the construction. As an adjective, it is welcome: You're welcome to try it, or You're welcome here for as long as you like. As a past participle of the verb to welcome, it is welcomed: I was welcomed at the door by the hostess.
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Both are correct with different meaning. In "Your suggestions are welcome at any time" welcome is an adjective. In "Your suggestions are welcomed by the committee" welcomed is the passive participle...
Both are correct, with slightly different meanings. In "feedback is welcome," which is the most common usage, welcome is an adjective describing feedback. In "feedback is welcomed," welcomed is a...