The “centring diphthongs” of the English RP/BBC accent are in continuous change. When I first studied English phonetics in the 1960’s there were textbooks which listed /ɪə/, /eə/, /ʊə/ and /ɔə/. It was made clear to me that the last of these, /ɔə/ was going the way of the dinosaurs, and indeed Daniel Jones remarks, in my edition of Outline of English Phonetics (Ninth Edition) “many speakers of Received English (myself among them) do not use the diphthong ɔə at all, but replace it always by ɔː”. This diphthong used to be very familiar to me, as I used to hear it in the speech of my older Lancashire (Merseyside) relatives born in the 19th Century. The diphthong /ʊə/ started its move towards the exit door rather later. Already by the time CUP took over publication of the English Pronouncing Dictionary, then on its Fourteenth Edition, the recommendation for words such as ‘your’, ‘poor’, ‘insure’ had /ɔː/, though /ʊə/ was always given as second choice.
This leaves us with /ɪə/ and /eə/. As many are aware, /eə/ is the next to go. Many recent writers have observed that the vowel element in SQUARE is more often a long pure vowel /ɛː/ than the traditional diphthong /eə/. Clive Upton’s Oxford Dictionary of Pronunciation dropped /eə/ completely in favour of /ɛː/, and later Alan Cruttenden’s Gimson’s Pronunciation of English (Eighth Edition) went the same way. Geoff Lindsey’s English After RP also uses /ɛː/. Until recently, however, Wikipedia’s articles on RP and on English Phonology stuck with /eə/ Recently someone changed the RP article so that the SQUARE vowel was described as a long mid front vowel, but was still symbolized as /eə/. This resulted in a very confusing description, so I have edited the article so that /ɛː/ (SQUARE) takes its place among the long pure vowels. I have also noted that /eə/ does not now belong in the list of diphthong phonemes. This is really just bringing WP’s practice into line with its statement that “The centring diphthongs are gradually being eliminated in RP”.
Now we have just /ɪə/. But Wikipedia says that “The remaining centring glide /ɪə/ is increasingly pronounced as a monophthong [ɪː]”. A reference to something written by me was cited in support of this statement, but I can find no trace of this statement in that article so I have removed the reference (I probably wrote something of the sort elsewhere, but I can’t remember where). However, Cruttenden (2014), p.154 gives a more solid support: “Increasingly, pronunciations with a monophthong [ɪː] can be heard within GB”.
I don’t believe the change from /ɪə/ to /ɪː/ has yet been completed, though I don’t doubt that the change is going on. In my own speech I believe I still have a diphthong in open syllables like ‘fear’, ‘near’, but I do have a pure vowel before /r/ as in ‘fearing’, ‘nearer’.
Watch this space! I will now have to go through other Wikipedia articles on English phonemes to bring them into line with what has been done for the RP article with respect to /eə ~ ɛ:/.