In my opinion they should be as relevant as ever. We are in the midst of the biggest peacetime UK negotiation, which they helped manufacture. But sadly many voters saw Brexit as a 1-day vote and then everything would be forgotten about, UKIP were very much associated with this and have been swept under the rug. They have become an outlet for the far-right and really just exist by thriving off former Conservative voters.
However, the UK does appear to be shifting slightly to the left in it's politics (not socialist at all like some people suggest, but it's moving to the centre ground again oppose to the centre-right where it's been for 5 or so years). Therefore the greater this shift, the more irrelevant UKIP become.
So to answer your question, in my opinion UKIP should have been relevant until the Brexit negations were over, then they would have been decimated by the Conservative broadchurch and by the left shift, but it appears due to their populist nature they have fallen as quick as they climbed.
By all accounts, they are not very relevant.