I had check out the 6.0.0-rc1-dev-1229
Look like the is no write access to dcm file anymore. I still see it read .dcm file form my library and open the correct locale datasheet file (the datasheet file specified as relative to the path of where the library file is). But there is no way to change it, or equivalent way for relative path with datasheet field.
I have custom global symbol library local in C:\my-kical-lib, but other people may store it at difference place, and link it their way.
The dcm file will definetly not be retired for version 5.1 as this version is not allowed to change anything on the file level. Current nightlies represent a development version that will become 5.1 which means the same holds true to them.
The dcm entry stuff is only used for information that can be different for aliases. The datasheet however can be added to the alias part and the symbol part. If the symbol field for the datasheet is left empty then the datasheet from the alias side will be copied into it when placing the symbol into a schematic. (This is new in version 5 and will stay for version 5.1)
This datasheet field should take path variables like any path field in kicad. So if you want to have your datasheet stuff portable then you might want to add a path variable that points to the place where you have your datasheets.
The interface for 5.1 will hide the distinction between the dcm entry stuff and the lib entry stuff by having everything managed in the symbol properties dialog. For the main symbol the information is now handled in the general tab. For aliases the respective fields can be entered in the aliases tab. (But to reiterate: It all behaves as if you add the information to the symbol properties dialog -> documentation tab in version 5.0.1. At least it did so a few weeks ago when i wrote my tutorial about symbol generation.)
If this changed then i will need to write an angry bug report again as such a change would break our test scripts again. (It would also create unnecessary noise on github which is totally unacceptable)
I think the issue (I saw this but didn’t realize it right away) is in the nighties there isn’t a separate text entry field for datasheet (like there is in 5.0.1) for the base/parent symbol, only the datasheet field property. There is, however, a space for the aliases in a nice table format. I’ll have to wait until I get home to generate screenshots for the nightlies as I only have 5.0.1-4 installed on my work computer.