Do you mean Opisthothelae as a node below iNat's Order Araneae containing Suborder Mygalomorphae and Suborder Araneomorphae, but excluding Suborder Mesothelae?

iNat uses WSC as its spider reference but WSC doesn't include nodes between order and family so those are maintained in this deviation
https://www.inaturalist.org/taxa/120474/taxonomy_details
and discussed in this flag
https://www.inaturalist.org/flags/495680

Currently beneath Order Araneae there are 3 suborders
Suborder Araneomorphae
Suborder Mesothelae
Suborder Mygalomorphae

Because Suborder Araneomorphae has nearly 2 million downstream observations it would be very disruptive to add a node between it and Order Araneae so even if there were a compelling case (see dicots discussion) it would be a large resource intensive undertaking to do. This doesn't seem to be a particularly useful node for identifying purposes which is the main use case for adding nodes. Remember you can search on multiple nodes (e.g. Suborder Mygalomorphae + Suborder Araneomorphae) thusly:
https://www.inaturalist.org/observations?taxon_ids=120474,84719

as I mentioned we don't want to add a node in between Araneae and Araneomorphae