It is not possible for a pope to abrogate the Traditional Mass. See Quo Primum.
Even if a pope could abrogate the TLM, and even if we assume arguendo that Bergoglio is pope, Traditiones Custodes did not abrogate the Traditional Mass.
Any excommunication for assisting at a Traditional Mass celebrated by a Catholic priest is invalid.
Just in case this post becomes personally relevant to you.
Saint Louis Catholic 
We so often use the Quo Primum argument.
But honestly, I don’t think you need it. When Pius V published it, it was unthinkable that any pope or council could possibly deep-six any ancient rite of the Church. And it should be unthinkable for us today, too.
And I think it’s telling that Paul VI could never pull the trigger, despite his utter desperation to leave nothing on the liturgical landscape but his reform. He even issued two indults for it!
“He even issued two indults for it!”
Could you link to this please?
Oh, you likely know of them, but just did not realize it:
1) Indult in Instruction Constitutione Apostolica, 20 October 1969. This was merely the (very stingy) exception granted, globally, to elderly retired Latin rite priests: “Elderly priests who celebrate Mass without a congregation and who might encounter serious difficulty in taking up the new Order of Mass and the new texts of the Roman Missal and Lectionary for Mass, may, with the consent of their Ordinary, keep to the rites and texts now in use. Special cases of priests who are infirm, ill, or otherwise disabled are to be submitted to this congregation.” In 1971, this was again modified to restrict celebration of the 1962 editio typica, “as emended by the 1965 and 1967 decrees.” So, it wasn’t even for 1962 as such, but only as modified by all of Paul VI’s modifications. Nonetheless, it was an indult for the traditional rite.
2) Indult of 5 November 1971, aka, the “Agatha Christie Indult,” protocol n. 1897/71. This indult was famously granted by Paul VI after he received a petition from numerous British luminaries, headed by novelist Agatha Christie. Like the first indult, this was limited to the traditional missal as modified by the 1965 and 1967 alterations, and geographically restricted to England and Wales. In practice, permission via this indult was often difficult to obtain from ordinaries, but nonetheless, it was an indult for the traditional rite.
This is by no means a defense of these indults, which were extremely stingy, often difficult to have recourse to during their time of legal effect, and pretty obviously conceived of by Paul VI as special, very limited in scope, and hopefully to become nullities over time as the Latin Church fully embraced the new Mass. As he insisted in a 1976 address, “The new Ordo Missae was promulgated in place of the old.” Still, nonetheless, it is worth recognizing that, however mutilated, the old rite was legally recognized as licit for celebration from Day One of the new liturgical regime. Even outside the chapels of the SSPX, it has never stopped being celebrated, licitly, in the Latin Church.
It is also worth noting what Annibale Bugnini himself says in his memoirs, that Paul VI was approached to promulgate a formal abrogation of the old Roman Rite, but declined to do so. It remains unclear why he did so. But it is another data point for the case for why it was (as John Paul II’s 1986 commission of cardinals concluded) never abrogated, and why, as I see it, it could *never* be abrogated, with or without Quo Primum. What Dave Gordon is arguing for this week is an impossibility.