When adding an opportunity and assigning a client I see that the last_updated timestamp field is updated which is super news, but I don’t see how to retrieve opportunities for a contact. I see end points for listing all opportunities and retrieving a single opportunity via opportunity id - either of these two are not practical for large data sets. Am I missing something, is there a way from contact result to find opportunities assigned to contact, or somehow query opportunities having specific contact assigned.
Is solution to search using email as search term obtained from contact object in list opportunities end point?
Which api are you using? There are limitations still with REST but with the xml-rpc you can query the Lead table to get all opps for a contact.
I am using the new API as they said to not use XML-RPC as it would be deprecated but with no specific date. I am using the user’s email to find opportunity which works well if emails don’t change. I have no idea what happens to past opportunities if someone updates their email.
So first, “who” said. Infusionsoft has been “saying that” for over six years and now that they’ve changed to two products it’s not going anywhere.
Also, there are some at IS that work with REST only and that’s what they continue to say to get people to use it.
In fact, they just rolled out an update for the xml-rpc which wouldn’t happen if they were getting rid of it.
I do know that while they say XMLRPC is not supported there are comments saying they will fix what’s broken but focus is on REST.
Plus I don’t think I could convince my fingers to code using XMLRPC 
A brief history of the API.
Infusionsoft API was introduced around 2006 based on the XML-RPC protocol. It means that has been running for 13 years now serving 1000s of scripts.
The REST API was introduced in or around 2016, so it is still relatively new in comparison. The problem is that it is incomplete, and does not match everything the XML-RPC API has. Infusionsoft are guiding developers to use the REST API instead of XML-RPC. Due to the incompleteness of the API, this makes it hard for developers to able to fully use REST. Depending on the script you are writing you could have complete REST API functionality. But there has been developers asking REST API questions in which turn requires using the XML-RPC API to fill in the gaps.
Although we do not know when the REST API functionality will be on equal terms of the XML-RPC API. For what I am seeing it is going to be into the 2020s before that is the case. The last API update was in October 2018, so no idea why there has been no updates for all this time.