Currently the AL language extension doesn't work because the compiler crashes on BC14 projects. You get the error message Cannot convert from to _MissingTypeSymbol (https://github.com/microsoft/AL/issues/7529) and it seems that nobody at Microsoft cares. Some people have pointed out in a Yammer discussion (https://www.yammer.com/dynamicsnavdev/#/Threads/show?threadId=2495228732121088) that BC 14 isn't even supported anymore, but many customers are still using it. With the C/SIDE development environment you didn't have this problem because you could site-by-site any CU version and use the same version the customer is working on.

So it would be nice if you could install the current and a specific version of the AL Language Extension site by site. Then runtime version 3.0 project should use the e.g. BC 14 AL Language Extension and the current version should run the runtime version 4.0 and later, so that BC 14 works as is and the new feature of the current version doesn't break the BC 14 AL Language Extension.