We used to have a template path setting for sites with MSM. Now that setting seems to be gone, how can we share a single set of templates used by two or more sites ?
There isn't an EE solution to this problem. When being able to set your template folder path for each MSM site went away, there was growing pains for all. But at the end of the day, if you have a good setup for your application, it is most easily solved at the deployment/devops layer of your setup.
The easiest way to handle this is to setup symlinks. For example, in your deployment process, you can ship all your templates to a single template folder, and then setup your EE MSM sites' template folders to be symlinks to that single template folder. It seems like extra overhead, but it is more of a "set and forget" fix, at least when deploying to Linux like servers. When you add new MSM sites, you will have to add the extra symlinks, but it isn't hours of work or anything.
Also, if you have a more full-service deployment system set up, you can also just write rules to deploy certain template folders to multiple folder destinations on the server you are deploying to. So that is a possible solution as well.
So yeah, a little bit of added complexity to achieve the same result, but not at all a mountain to climb (at least on Linux based servers, can't say the same for deploying to Windows based servers).