I'm aware of the gencode annotation, and RefSeq annotation. Those do a good job at providing gene annotations & models, showing where all the known exons / introns are. In practice, these types of annotations often mean presenting for each gene:

a collection of intervals that go together to form a gene, and some sort of meaningful tag/annotation for each of these intervals

Based on recent studies of splicing, what is available out there (supplementary materials / annotation files / databases) that do not only list intervals + annotations for a gene but also show alternative splicing structure? Present all the observed transcripts / isoforms in one annotation?

Thank you!