The decline of relativistic mass

Vesselin Petkov notes how the concept of "relativistic mass" has gone out of fashion:

These facts make the campaign against the concept of relativistic mass both inexplicable and worrisome. Instead of initiating and stimulating research on the origin of relativistic mass (and on the nature of mass in general) in order to achieve a more profound understanding of this fundamental concept in physics,7 the relativistic mass is not mentioned at all in many publications8 (see, for example, the well-known textbook [35]) or, if it is mentioned, it is done to caution the readers9, that "Most physicists prefer to consider the mass of a particle as fixed" [25, p. 760], that "Most physicists prefer to keep the concept of mass as an invariant, intrinsic property of an object" [32], that "We choose not to use relativistic mass, because it can be a misleading concept" [36] or to warn them [22, p. 1215]:

Watch Out for "Relativistic Mass"

Some older treatments of relativity maintained the conservation of momentum principle at high speeds by using a model in which a particle's mass increases with speed. You might still encounter this notion of "relativistic mass" in your outside reading, especially in older books. Be aware that this notion is no longer widely accepted; today, mass is considered as invariant, independent of speed. The mass of an object in all frames is considered to be the mass as measured by an observer at rest with respect to the object.

As he explains, this opinion is pretty arbitrary, and relativistic mass is analogous to length contraction or time dilation. Yes, it depends on the frame, and it can be a little confusing, but that's relativity.