I've never seen any commentary suggesting John 20:28 has been altered, either by intent or carelessness, but 1 Tim. 3:16 certainly has.
As a footnote to 1 Tim. 3:16 says at a USCCB (UNITED STATES CONFERENCE OF CATHOLIC BISHOPS) web page,
“7 [16] Who: the reference is to Christ, who is himself “the mystery of our devotion.” Some predominantly Western manuscripts read “which,” harmonizing the gender of the pronoun with that of the Greek word for mystery; many later (eighth/ninth century on), predominantly Byzantine manuscripts read “God,” possibly for theological reasons.”
www.usccb.org/nab/bible/1timothy/1timothy3.htm
Note that well....” many later (eighth/ninth century on), predominantly Byzantine manuscripts read “God,” possibly for theological reasons.”
I said is 'could have been'. But, then, there is no commentary on the addition of the Father, Son, Holy Ghost in Matthew 28:19 either, except that, unlike the case of John 20:28, there is indirect evidence that at least a variant version did exist because the verse is quoted by Eusebius (later 3rd century) no less than 17 times without the tirnitarian formula.
Predictably, he quotes Matthew 28:19 five more times after the first Nicene Council (which established the triniatiran dogma) and in all five cases he includes the triniatrian formula. I guess he suddenly must have found the other version/s.
The fact that there are numerous other add-on trinitarian verses known in the New Testament certainly opens a realistic possibility that John 20:28 could be another example of one given it's strongly suggestive tirnitarian character.
Note that well....” many later (eighth/ninth century on), predominantly Byzantine manuscripts read “God,” possibly for theological reasons.”
Well, certainly, because the source of the fraudulent entry is none other than the 5th century Codex Alexandrinus, the official Bible of the Greek Orthodox Church, against which these manuscripts were made. The fraud involves only changing the capital omicron O to capital theta by placing a short horzontal line through it, Θ.
The reason KJV verse reads "God" isntead of "He" is because Textus Receptus, which is the basis for the Engllish translation, was obtained through unreliable 13th century Byzantine manuscripts.
This is a perfect example how a fraudulent entry in one manuscript ends up poisoning all manuscripts and translations derived from it.
Just look at how many different versions of the Greek Old Testament (Septuagint) we have (mostly 1st century AD work by Jewish scribes); then there are two version of Syriac Old Testament, one based on the Greek Septuagint (Syr-Hexpla), and the other based on the Hebrew Masoretic Text (Peshitta).
Needless to say, there are variants of the variants on top of that, all hand copied freelance style and translated in a like manner.
Apostle Paul must have been very cynical when he wrote "God is not the author of confusion," (I Corinthians 14:33)