Is there any New-WebServiceProxy alternative?
I'm using currently PowerShell 7 and the lack of New-WebServiceProxy affect me in several Scripts that was running with this CmdLet as expected in PS 5.1.
As 'danno' answered before, Invoke-WebRequest is the natural option in Core editions.
Googling I found two sites with examples to implement a workaround for SOAP Web Services:
https://www.itprotoday.com/powershell/getting-started-soap-based-web-services-and-powershell
... and ...
https://www.powershellbros.com/send-soap-message-powershell/
Maybe someone says it that this is go back, but really works!
A combination of ConvertTo-XML with Invoke-WebRequest should be able to get you where you need to be, both of which are supported in PowerShell 6.