Thanks for question-- I cannot prove it. I do believe that the alleged abdication of St Celestine was(is) illegal. Pius X has proclaimed Boniface  6 & 7 to be anti-popes and MO is that there may be one more to go.

History Of Popes: 1830-1914 by Prof Owen Chadwick pg 403.

.....' and Pius X had to get rid of four popes from the past---Boniface VI(6), who in 896 was illegally pushed into the see; Boniface VII(7), who in 974 strangled the emperor's ambassador, ...........'

I cannot recall offhand but I believe the yr was '06 or '07.

U are incorrect sir. I would advise researching the various Catholic Encyclopedia's that are avail.

Fr Radecki's book reprints the list from the National Catholic Almanac of 1967. While the list is incorrect re: the Fr popes of GWS and Boniface 6 is missing, it does list Boniface 7. I believe Chadwicks book. If U don't that is your own business. BTW-- other research has been posted here in the past informing us that the yrs Boniface 6 & 7 were declared anti-popes was 1906 & 1907.

If U check, u will find that the various Catholic Encyclopedia's designate them as anti-popes. This has all been discussed b4.

I believe what I have posted to be true.

Are U denying that Boniface 6 & 7 are anti-popes?

The Catholic Encyclopedia defines an antipope as, "A false claimant of the Holy See in opposition to a pontiff canonically elected." It lists only 1 claimant of the name Boniface as an antipope-- Boniface VII, who reigned in the year 984.

Boniface VI-- A Roman, elected in 896 by the Roman faction in a popular tumult, to succeed Formosus. He had twice incurred a sentence of deprivation of orders, as a subdeacon & as a priest. At the Council of Rome, held by John IX in 898, his election was pronounced null. After a pontificate of 15 days, he is said by some to have died of the gout, by others to have been forcibly ejected to make way for Stephen VI, the candidate of the Spoletan party.

Boniface VII (antipope)-- A Roman...was intruded into the Chair of St. Peter in 974; reinstalled 984; died July, 985. In June, 974, 1 year after the death of Emperor Otto I, an ιnѕυrrєcтισn...(occurred) at Rome, during which the Romans threw Benedict VI into the Castle of Sant' Angelo, & elevated as his successor the Cardinal-Deacon Franco, who took the name of Boniface VII. The imprisoned pontiff was speedily put to death by the intruder. But in little more than a month the imperial representative...had taken possession of the city, & Boniface...fled... After an exile of 9 years at Byzantium, Franco, on the death of Otto II, 7 Dec., 983, quickly returned to Rome, overpowered John XIV (April, 984), thrust him into the dungeons of Sant' Angelo, where the wretched man died 4 months later, & again assumed the government of the Church. ...For more than a year Rome endured this monster steeped in the blood of his predecessors. But...after his sudden death in July 985...the body of Boniface was exposed to the insults of the populace, (&) dragged through the streets of the city...naked & covered with wounds...

Boniface VIII-- Born at Anagni about 1235; died at Rome, 11 October, 1303. On the 13th of Dec. 1294, the saintly but wholly incompetent hermit-pope Celestine V, who 5 months previously...had been taken from his obscure mountain cave...& raised to the highest dignity in Christendom, resigned the...papacy. The act was unprecedented & has been frequently ascribed to the undue influence & pressure of the designing Cardinal Gaetani. 10 days after Celestine V's gran rifuto the cardinals went into conclave...& on 24 Dec. 1294...elected Cardinal Benedetto Gaetani, who took the name of Boniface VIII.

The Catholic Encyclopedia is not the end of knowledge, but the beginning of it. Acting from, this beginning, if you're interested, then research the original texts & discover whatever you may.

To be clear--- I am not sure that Chadwick( as a non- catholic) understands exactly what an anti-pope is. He uses the term 'get rid of'.