I guess my follow-up question would be this: haven't there been theological controversies since 451 that need an ecumenical council to resolve them? It seems like potential influence from later EO and RC thought like absolute divine simplicity or the essence-energies distinction would have been accepted by some and rejected by others within the OO Church.
I have heard my own bishop talk approvingly of the essence-energies distinction, although he seemed to also recognize that talking about it in exactly those terms is a later development, and 'foreign' in the sense that it did not come about via some debate that actually occurred within our own church. I don't know what the other thing is.
Those may be bad examples since they didn't need councils considered ecumenical in their own churches to become effectively universally accepted, but I do assume there must have been some kind of proposal made in the last 1500 years that was controversial.
I think they're actually great examples, because as you acknowledge, they show that an ecumenical council is not needed in order to be accepted. All of our controversies have been similar: the Syriacs were out of communion with the Armenians for a time over some of the eucharistic practices of the Armenians, and they handled it themselves; sections of the Ethiopian Church came under the sway of the Catholic Portuguese for about a decade in the 17th century, and they handled it themselves; etc., etc. The things that have required other churches within the communion to settle have had mostly quite clear and obvious ways forward, like the aftermath of the Ethiopia-Eritrea war in the early 1990s, when Eritrea was now an independent country, but the Ethiopians didn't want to let it go (ecclesiastically-speaking...or I suppose in other ways, too). The bishops of the Church in Eritrea went to seek autocephaly from the Pope of Alexandria, no differently than how the Ethiopians themselves had gained it some decades earlier (albeit without the war). Even today, when there is some theological dispute going on in the Ethiopian or Eritrean churches, it is common for them to appeal to nearby Coptic Orthodox bishops to provide consensus concerning what the correct teaching is. This makes perfect sense, as the Ethiopian and Eritrean Orthodox Tewahedo churches are the daughters of the Coptic Orthodox Church.
One place where this kind of doesn't hold is the messy and sometimes very violent schism between the Syriac Orthodox 'Jacobites' and the Malankara Orthodox Syrians in India. The former are (and consider themselves to be) an integral part of the wider Syriac Orthodox Church, currently under the guidance of HH Moran Mor Ignatius Aphrem II (though the Syriac Orthodox Indians have a Catholicos, currently HB Maphrian Baselios Thomas I), while the latter are autocephalous, currently led by their own Metropolitan who is
also styled as Catholicos, HB Baselios Marthoma Mathews III. In recognition of the fact that both hold the same faith in the three ecumenical councils, and that this is a jurisdictional mess that needs to be sorted out by the Indians themselves, we in the Coptic Orthodox Church (and to the best of my knowledge, also the Tewahedo and the Armenians) recognize both, and will commune both. It is a tragic schism, but not something in which any of us apparently see any wisdom in taking sides. While this may seem unwise, it is both an authentic reflection of how we've pretty much always functioned, and also a testament to the hope that we continue to have that they
can be reconciled to one another, as it wasn't too long ago that both came together to elect one Catholicos to govern both of them (it was in the 1970s, and I don't really know the details of why it didn't last, but the point is it wasn't a million years ago). While their schism from one another was slightly newer than that, the Ethiopians recently reconciled with their own 'synod-in-resistance', which had sprung up among some in the Ethiopian diaspora who objected to the elevation of HH Abune Paulos in the early 1990s (a time of political chaos, with the ending of communism in Ethiopia), so there is precedence for this in the very recent history of our communion (this reconciliation was only in 2018).
