This is an interesting find ^[1]: a document has been uncovered in which Pope St. Pius X tolerated communicatio in sacris (i.e. shared communion) with the Orthodox.

Translation:

Most Blessed Father!
Andrew Szeptycki, Metropolitan of Halycz, Metropolitan of Kiev and Administrator of all Russia at the foot of His Holiness most humbly asks that faculties may be conceded to himself and also to confessors in communion (capable of being communicated) for dispensing secular faithful from the law which forbids communicatio in sacris with the Orthodox as many times as they will judge it in conscience to be opportune.

Our Most Holy Father Pope Pius X deigned to sign with his own hand this document written by me with the words “May be tolerated”.

A number of years ago I wrote a paper on pre-Vatican II ecumenism in the Catholic Church ^[2]. Many people are under the (false) impression that Vatican II “started” ecumenism in the Catholic Church, but my research showed me that while Vatican II was a giant leap for the Church in regards to ecumenism, it was the result of many years of ecumenical work within the Church.

Note: the purpose of this post is not to endorse a wider practice of communicatio in sacris with the Orthodox, but to emphasize that the pre-Vatican II Catholic Church did not consider the Orthodox to be simply schismatics outside the Church – which is the impression you get in some quarters.