That of course depends on what it was for. If it was when he/she was five and has not gone back since and doesnt need to, then it should be fine. If he/she is still going or is on psychiatric medication, then I would need to think twice about it. I guess it all depends on the person’s specific case. Anger issues are very relevant in marriage, and it all depends on the duration, intensity and reasons/background for why getting therapy.
I think that all therapy received should be said before the engagement, especially if medication was involved (even if off of it), but at first only “important” therapies need to be told. It all must come out in the wash way before the lechaim.
Dont stop going just for shidduchim. Go if you need it but just tell the other side when it is time. if you are up front about it, the family might not make such a big deal about it but if it is kept a secret and is only told about much later, then it might be a major factor.