Were there universities that treated socioeconomic status as a major factor in college admissions?

Historically the top universities in the UK, Oxford and Cambridge, admitted an overwhelming majority of entrants from "public schools" - meaning in this case fee-paying private schools. Public schools are affordable only by very wealthy socioeconomic classes.

During the 20th century the admittance rate went from "overwhelming majority" to "majority" and has now reached the point of "disproportionately more" - meaning that a far higher percentage of students from expensive schools are accepted than the percentage of students in private schools. (Specifically around 35% of UK admissions are from public schools, while around 7% of UK students are educated at private schools.)

One might with some justification say the elite students are more likely to be educated at a fee-paying school, and so a disproportion is to be expected. However in the early part of the 20th century it is certain that being from a public fee-paying school - which correlated extremely well with high socioeconomic status - gained you a significant advantage.

East Germany founded a number of Arbeiter- und Bauern-Fakultäten ("colleges for workers and farmers") after WWII in order to make students from non-privileged backgrounds ready for university. Totally anecdotic evidence (Heiner Müller's poem Das Duell) suggests that some faculty were not convinced such a scheme could work. But it seems to have worked, at times anyway.

Most of these colleges were dissolved in the late 1950s, after a generation of students had gone through socialist schools. From that point on, class-based affirmative action meant that children with working-class backgrounds had better chances to be admitted into upper secondary school which would later enable them to go to university.

Notably that was at the expense of children from family backgrounds that were deemed bourgeous (i.e. children whose parents had graduated from a university) or politically unreliable (e.g. children from Christian families, if they were not openly supportive of the authorities).

Obviously by that point these affirmative action policies could be understood as an attempt by the government to discriminate against groups of people it was suspicious of, rather than helping those that had historically been at a disadvantage.

In West Germany, it seems as if it was assumed that getting students from working-class backgrounds to university was mainly an economic problem, and student grants and loans were implemented to address (possible) economic problems of a large share of the student population.

The workers movement, starting in the 19th century, always critizised the lack of oportunities for workers to get a higher education. When they gained enough following to stand on financial sound footing, workers unions started to found educational institutions, among them also institutions of higher learning. They had the double goal to provide workers with access to more knowledge, and to train future union leaders and functionaries in fields like economic sciences, political sciences, labor relations and labor law.

I cannot give an international overview, but for Germany, here are some institutions of higher learning that were geared at workers that did not have the level of secondary education (Abitur) to access a university, but had professional training or experience.

(The German educational system was and partially still is especially rigid in blocking access to educational oportunities. Until the end of the 20th century, children age 10 were separated into three different sorts of schools that would largely preordain their future opportunities: Volksschule/Hauptschule was aimed at future factory or craft workers, Realschule at future commerce or office workers, and only the Gymnasium and its exam, the Abitur, gave the unconditional right to enter a university.)

• Akademie der Arbeit, Frankfurt was founded by the German Trade Union Confederation (DGB) and other unions in 1921 and was associated to the Frankfurt University, with approval of the Prussian state. After its abolition by the nazi regime in 1933 it was refounded in 1946 and still exists today.
  Personal note: my father was a student there in 1947. He had left the Volksschule in 1939 aged 14, finished vocational training as a metal worker in 1941 and was drafted for service in the war 1943. He used his education at the academy to enter government service in a mid-level career that otherwise would only have been accessible with a Realschule exam.
  Martin Allespach, Rainer Gröbel (Hrsg.): 100 Jahre Europäische Akademie der Arbeit. Eine Institution für Lehre, Forschung und Mitbestimmung. Frankfurt am Main 2021

• Sozialakademie Dortmund was founded in 1947 after the model of the Frankfurt academy under joined stewardship of the state of Northrine-Westphalia, the city of Dortmund and the DGB.
  Ludwig Bußmann, Bärbel Malkowski-Andrzejewski, Rolf Ruppio: 40 Jahre Sozialakademie Dortmund: eine historische und empirische Untersuchung, Duncker & Humblot, Berlin 1990

• Akademie für Gemeinwirtschaft, Hamburg was founded in 1948 by a group of trade unionists and social democrats. It is now a part of the Hamburg University.
  Bärbel von Borries-Pusback: Keine Hochschule für den Sozialismus : die Gründung der Akademie für Gemeinwirtschaft in Hamburg 1945 - 1955, Opladen: Leske und Budrich, 2002

• Hochschule für Arbeit, Politik und Wirtschaft, Wilhelmshaven existed from 1949 to 1962. It was a university of the state of Lower Saxony, but especially tried to attract students with a professional background and/or no Abitur.
  Oliver Schael: Von der Aufgabe der Erziehung. Das gescheiterte Reformexperiment der „Hochschule für Arbeit, Politik und Wirtschaft“ in Wilhelmshaven-Rüstersiel (1949–1962). In: Detlef Schmiechen-Ackermann, Hans Otte, Wolfgang Brandes (Hrsg.): Hochschulen und Politik in Niedersachsen nach 1945. Göttingen 2014, S. 53–79.