The three dimensions of length are altered by MASS-ENERGY just as surely as the fourth dimension of time; in special and general relativity, the only difference between length and time is their opposite signs in the distance metric.
For example, in the Schwarzschild metric (the solution to the Einstein field equations outside a spherically symmetric mass distribution) the coefficients of both radial distance and time are different than in the absence of mass: https://en.m.wikipedia.org/wiki/Schwarzschild_metric
Of course length is altered. But length is different than space. Which is why Einstein always spoke of “length contraction” rather than “space contraction”.
When Einstein talks about length being altered, he is speaking about the apparent change in length, of measuring rods and such, to the observer in a moving inertial frame. He never talks about the actual 3 spatial dimensions in the universe in reality being “warped” as some modern day interpreters of Einstein do. Only the measurements, or our perceptions of length are being altered by motion, that is what Einstein talks about.
Your measurements of length are certainly altered but space itself is not altered. For example, if you travel from here to Alpha Centauri at a relativistic velocity, the length between Earth and Alpha Centauri, to YOU, appears to shorten. However, the space between Alpha Centauri and Earth is not actually changing, as anyone who measures it from either Earth or Alpha Centauri, or any other point in the universe can confirm. Only YOUR measurement is changing.
I’m not disputing that when you do the math that time and the spatial dimensions are interchangeable. However, these are just mathematical conventions. In reality, time and space are clearly different things, even if we can conceptually represent them as interchangeable axes in a coordinate system for our own convenience. Einstein’s entire focus in relativity was solving the problem of SIMULTANEITY in moving systems, which is a temporal problem, not a spatial one. His use of Minkowski space to unify the mathematics of space and time to solve the problem works, but it has the unfortunate side effect of creating this confusion that time and the spatial dimensions are interchangeable IN REALITY, when they are really only interchangeable in this limited mathematical framework.