We were formed long before 1988 and I don’t think we were in the LCA prior to that.
Get a good lawyer.
It may be that your Synod is trying to assert a claim that by having accepted ELCA clergy and that by having paid ELCA Mission Support (benevolence) you were under the “Model Constitution for Congregations” EVEN IF YOUR CONGREGATION HAD NEVER FORMALLYH ADOPTED THAT DOCUMENT and is asserting claim against the congregation’s property based on the Model Constitution.
The Episcopal Church has spent millions litgating against its departing congregations with mixed success for over a decade. The ELCA may be starting to follow the lead of its “Full Communion Partner”.