Maybe it's not universal but in school literal equation basically meant there were letters instead of numbers.

It's the term we use for instance when going from the equation of a line like y=3x+2 to lines in general y=ax+b (a and b in ℝ)

And i agree it's a lot better to specify to solve for x (because you can solve for anything or have multiple variables).

Although x being a variable, and solving for it would be the most logical assumption.

I teach secondary and postsecondary math courses. The term "literal equation" was used in Texas where I taught for 17 years. The Algebra 1 state standard A.12E says that students are expected to "solve mathematic and scientific formulas, and other literal equations, for a specified variable." I also taught college undergrad courses in Texas, including College Algebra, and I don't recall ever seeing the the term used there, but I used it in class because my students were familiar with it. Now I teach in Oregon, and the term is not a part of this state's standards from what I can tell.