Which Christian denomination removed trinitarian language from the hymn "Holy, Holy, Holy"?

This link gives several variations of this hymn's lyrics, without mentioning how they came to be.

As a member of the Church of Christ I was once in a congregation which used a hymnal in which the lyric "God in three persons, blessed Trinity" had been changed to "God over all, and blessed eternally".

The hymnals we use tend to be compiled by a member of the Church, and at times are edited for doctrinal reasons, but I don't know why anyone from the Church of Christ would make such an alteration. I've not heard of a congregation which rejects the Trinity. It's possible that whoever compiled the hymnal in which I saw this variation had copied it from the hymnal used by some other faith group. The alteration does not turn the affirmation of the Trinity into a denial; it simply makes the hymn silent on the matter, and so it would not conflict with any of the Church's teachings.

It is my understanding that the pioneers of the Seventh-day Adventist Church, who were as non-trinitarian as were the Jehovah's Witnesses who split out from them, altered the lyrics to that hymn to remove the Trinity references. The reason documentation for this is extremely difficult to find is that modern Seventh-day Adventists are officially Trinitarian. Their newer hymn books have the Trinitarian lyrics, and this earlier history is concealed.

Speaking of the Trinitarian line in "Holy, Holy, Holy", THIS SITE (which details some other Trinitarian hymns as well), says:

The 1909 and the 1941 Adventist version read as follows: “God over all who rules eternity!”

I haven't found any old versions, and in particular not Holy, Holy, Holy, but the introduction to the Worldwide Church of God's Hymnal freely admits that they changed lyrics when necessary:

When we moved to Pasadena to found Ambassador College, in 1947, I asked my brother to devote full time to setting the words of Psalms — and/or any other scripture — to music in the four-part harmony style of hymns. For some little time the Church, then small, sang the first 12 or 15 hymns that had been composed. The Church grew, and so did the number of hymns sung with God’s own inspired words. When we were able to print our first Church of God hymnal, not yet having a sufficient number of our own new hymns we filled out our comparatively small hymnals with well-known Protestant hymns whose words were not non-scriptural — even having to change the words in a few instances. Gradually through the years succeeding editions of our hymnal have contained fewer and fewer of the old Protestant hymns, and more and more of those composed by my brother. For the past few years our congregations have been singing our own hymns almost entirely. Now, at last, the time has come when we can omit the old Protestant hymns almost altogether, with more new hymns, and a few others whose words are proper, which we feel our congregations would want to sing occasionally. It is, indeed, a happy achievement to have, at last, God’s own hymnal for God’s own Church. It is a happy event, also, that we now produce the hymnal with hard covers, and make them available to members to have in their own homes. I feel this is another milestone for God’s Church

— HYMNAL_EN_E01.pdf

A non-Trinitarian denomination certainly could not 'allow' congregational singing of the line, "God in three persons, blessed Trinity". As for the comment in an answer about "the Jehovah's Witness hymnal", please note that they refuse to call their songs 'hymns'. They do not use any hymns from any other denomination, and for the past 60 years at least only use their own songs. They are avowedly anti-trinitarian. They would never, ever sing the "Holy, Holy, Holy, Lord God Almighty" hymn, even with that trinitarian line replaced with a neutral one. They could never have been responsible for that line being changed, because they wouldn't touch such hymns with a barge-pole.

A possible candidate for changing that line could be Unitarian denominations, or perhaps the Worldwide Church of God (prior to it coming over to trinitarianism about 30 years ago).

If a process of elimination helps you find the answer, here are three facts:

The Church of Scotland Hymnary (3rd edition) reprinted in 1974, had that line.

The Baptist "Hymns of Faith" collection reprinted in 1985, had that line.

The Evangelical "Mission Praise" collection reprinted in 1990, had that line.

Given the way the ecumenical movement has influenced many denominations, however, I would not be surprised if the Church of Scotland, Anglican and Episcopal churches have in more recent years dropped the clearly trinitarian line in favour of the non-trinitarian one. Once copyright no longer obtains (Reginald Heber having died in 1826) anyone can take liberties with their lyrics. So, who was responsible for the initial tampering? That remains such a good question, I shall investigate further and hope to add an edit to this answer, in order to satisfy you.

Reginald Heber (21 April 1783 – 3 April 1826) was a Church of England parish priest who became Bishop of Calcutta in 1823. Heber wrote "Holy, Holy, Holy" for Trinity Sunday, a day that reaffirmed the doctrine of the Trinity and was observed eight Sundays after Easter. The Greek phrase Trisagion translates as "Thrice Holy" – as in this hymn God is described as holy in three different qualities; Agios o Theos means "Holy God". The hymn was first published in 1826.

Heber wrote 57 hymns which were collected by his widow and published in his poetical works in 1842. The tune “Nicæa” was written by Dr. John B. Dykes, a renowned Doctor of Music.

The original hymn (written by Heber) contained four verses, with the first and the fourth ending with “God in three persons, blessed Trinity!” Source: https://www.hymnologyarchive.com/holy-holy-holy

However, another source shows only three verses adapted from Heber's original, with “God in three persons, blessed Trinity!” given only in the first verse: https://www.hymnal.net/en/hymn/h/6

In support of the answer given by brasshat, I found an official Christian Science link which shows how the original words have been changed:

From The Christian Science Hymnal, 1932 edition:

Holy, Holy, Holy, Lord God Almighty, Early in the morning our song shall rise to Thee. Holy, Holy, Holy, merciful and mighty, Which wert, and art, and evermore shalt be.

Holy, Holy, Holy, darkness cannot hide Thee, Though the eyes of sinful men Thy glory cannot see. Thou alone art holy, there is none beside Thee, Perfect in power, in love and purity.

Holy, Holy, Holy, Lord God Almighty, All Thy works shall praise Thy name in earth, and sky and sea; Holy, Holy, Holy, merciful and mighty, Which wert, and art, and evermore shalt be.

Source: https://hymns.plainfieldcs.com/hymn-117/

As can be seen, the Christian Science version, whilst retaining the words “Holy, Holy, Holy,” removes all reference to “God in three persons, blessed Trinity!” which appeared in Heber’s original, four verse hymn.