There’s no doubt the Clinton impeachment was polarizing; it was also counter-effectual for the Republicans. In the middle of impeachment, during the November 1998 elections, the Democrats gained five House seats.
I’d say there’s a lot of doubt about that statement, given that a year later, while Clinton himself remained broadly popular, his conduct colored the 2000 Presidential election, to the point where Gore actively distanced himself from Clinton, and Clinton’s honorability was a focal point of Bush’s campaign. By the start of Bush’s term, the Republicans held the House, the Senate and the Executive Branch, and they remained in control until 2006.
That doesn’t sound like Impeachment hurt Republicans politically. It sounds like, if we connect political outcomes to Clinton’s impeachment, that following Impeachment, Republicans remained in partial control of the government for two years following, and then held total control for six more years after that.
I don’t know where the mythologizing began that Clinton’s Impeachment hurt Republicans, but to me, this idea is clearly a myth.