No, that’s not true, anything can be repealed or changed except for one thing
“No state may without it’s consent be deprived of it’s equal suffrage in the Senate.” Meaning a proposal to give Cali a third Senate seat (which has been suggested by libs) would require ratification by all 50 states.
But an amendment to give each state a 3rd seat would only require the usual 2/3s of Congress and 3/4s of States to ratify because 3 each is sill equal. Cutting it down to 1 each also still equal.
As you may recall, I would support an amendment to give each state a third Senator (and set the national size of the House at 3 Representatives per Senator, which, with 50 states, would be 450 Representatives). That way there would be a Senate seat up every two years in every state, thus giving voters in all states the ability to participate in both House and Senate elections every two years. And setting the aggregate number of Representatives at three per Senator (i) eliminates possible hanky-panky by a future Congress and (ii) returns the ratio of Representatives to Senators to one close to the one original set by the Framers, thereby giving increased weight in the Electoral College to candidates who win more states. In 2016, Trump would have won 336-253 and in 2000 Bush would have won 301-288, in each case with 295 electoral votes needed for a majority).