Refer this documentation: https://docs.alfresco.com/5.2/pra/1/concepts/cmis-bindings.html
It says that there is SOAP webservice binding available via CMIS. Check to see if it works for you. I am not sure if it even works.
I worked with a similar requirement couple of years ago and i choose to write a SOAP webservices middle layer between Alfresco repo (AlfrescoOne 126.96.36.199) and third party consumer, where third party consumer (in your case the legacy system that can consume and send only SOAP request/response only at this moment) sends the SOAP request to the middle layer, middle layer parses the request and then talks to alfresco repo over the REST (OOTB REST APIs and custom develped) and returns the SOAP message back to consumer.
So may be you can try similar approach as an alternative if its viable for your business needs.
Steps to follow:
1- Get the WSDL that DMS endpoint produces and used by the legacy system at present.
2- Implement a middle layer SOAP webservice. You can either use axis2 or jaxws and develop a spring boot app. Check some useful links below:
Developing soap webservice using spring boot:
Apache Axis2 SOAP webservice:
Sample soap webservice project: https://github.com/abhinavmishra14/soap-demo
3- Using the wsdl (as per step 1) develop the SOAP endpoints in the middle layer application in accordance with the legacy system's expectations in terms of request and respose it needs. Middle layer will get soap request, parse the request and then call Alfresco's rest apis, prepare the soap response upon response from alfresco's rest apis and return the soap response back to legacy system.
4- Deploy the middle layer on same host where alfresco is deployed or on different host making sure middle layer is able to connect to alfresco. Test the services via soap client/soapui etc.
5- Integrate with legacy system
(ACSCE, AWS SAA, Azure Admin)