word "suggested" for already accepted proposal
Yes, suggested just means that it a suggestion was made and does not imply that the suggestion was accepted / rejected or not. In other words, regardless of whether the suggestion is accepted or rejected, or if no decision has been reached yet, it has still been suggested.
There is a slight problem with your example sentence. You seem to be missing an article:
I suggested a tag synonym on this; the suggestion got a few votes and was accepted.
Yes.
You can say this as long as it was suggested then, which is what the word means.