Tough to get a straight answer from RC or EO on this; because of infalliblity questions I suppose.
The evidence is that the decisions of the local synod of Laodicea were accepted as binding on the whole Church of the time.
As I pointed out to you earlier, infallibility is a Western concept.
We receive the canons of the councils as canonical, which means as 'the rules.' Rules are neither immutable nor infallible. There is a canon, 14 I believe, from the 7th EC that declares that all Readers and Acolytes in the Church must always go about in public in their rassa, their black robe/vestment. I don't.
Likewise, the anti-Rabbinic statements of some ECs, and the overall anti-Rabbinic sentiment of the declarations found in several councils are steeped in historical context that is not profitable to explore here. Suffice to say, the East was far more inclined to hold onto or be friendly with Semitic concepts. Many of the practices later deemed heretical (quartodeciman) began in the East.
We still refer to Saturday as Sabbaton, and liturgies are done on Sabbath. As to whether working on Saturday is required- seriously? Who does. This issue is moot in our culture.
What is not moot is the legacy of unused and uneneforced canons. They should be repealed if not seen as rule- or if they are, why. But we don't hold ECs since the schism.
I observe my day of rest on Saturday, and day of worship on Sunday. The canon is foolish and frivolous.
One of the reasons EO and RC members are reluctant to speak on this matter or others involving ECs is because they don't feel historically or theologically prepared to do so, any more than a typical Protestant is prepared to give an able defense of sola scriptura.
Are Sabbatarians heretics? By ancient definition, one who choses, yes. Moral definition is quite another thing.
If they were honest, they'd admit seeing us that way- captives of Rome and sol invictus- so let's not pretend offense. I think both sides are frivolous and self-aggrandizing.