Richard, thanks for an interesting post.

I don't think I would normally say 'It is I who . .' or 'It is me who . .' Both sound overblown and a bit pretentious.

I would say 'I did it' or 'I did that', unless, of course, I wanted it to be overstated.

Is this an example of how grammatically correct archaic forms can fall in to disuse and become obsolete, to the point of sounding strange ?

"It's I" is correct but a bit dated. "It's me" is more common. We were given the subject object rule at school, where I is typically the subject and me is typically the object - but with exceptions such as this. The previous sentence is also one of those relatively rare cases where it's correct to say <em style="color: rgb(0, 102, 204);">I is</em> and <em style="color: rgb(0, 102, 204);">me is</em> with or without quotes.

I think I've only seen<em> It is I..</em> in some supposedly-formal or poetic situations, so I'd only use them ironically.

I didn't know about linking verbs, but I haven't found any other that can be used with either subjective or objective pronouns, I mean they doesn't sound right.

At the beginning of my English studies, I was always perplexed by phrases like "It is me". Because if you translate directly the pronoun "me" into Russian, it corresponds to the form of the dative case, and the pronoun " I " to the form of the nominative case. In our language, these are different grammatical forms and "I" can never be replaced by "me", otherwise people will not understand you. In English there are no cases, but before they were. So I think there was a transformation of I - >me. From my point of view, the phrase "It is me" sounds more melodic than "It is I" and it is easier and faster to pronounce. That's the reason.

I used to say this phrase as "It is I" just translating it as a carbon copy from my native language, but after I heard in the movie "Liar, liar" the famous phrase "It was me!" then my opinion has changed :)

I just take the wife out of the equation and find as you did that it is either I or me depending on the sequence.