First up I'll say I do not know for sure the purpose of this touch file. As you have discovered all seems to be okay without it. The requirement for this touch file has been around ever since BYO media servers were allowed to have more than the single mount point.
It may be that as the usage of pool is probably low at present, only one mount point is being used, so the lack of the file is not a problem. I would see no problem creating the touch file and restarting NetBackup on the media server. Try running the command "/usr/openv/pdde/pdcr/bin/crcontrol --dsstat 3" and see what shows for each mount point.
Also I think it may be required for is when using some of the MSDP utilities (e.g. msdpcheck etc.) that the existance of the file is required to ensure all mount points are checked (that's my best reading of some internal articles).
Really though you should contact support and ask for advice.
Please report back if you do get an answer.
As David suggests, Support is probably the best option for this. The only thing I can add to David's comments is one Vox post from 2017, that references CRCONTROL possibly works differently, or has different arguments if the file is missing. See TRV's comment, which was marked as the solution:
Hope this helps,
In the past you were not able to add a second partition to a MSDP pool unless the file was there. You got an error. But you were able to create very large partitions and run into problems that way. Just this week I saw a client with a 90 TB partition.
If your pool is UP and running, backups working I don't see a reason to worry. Add the file just to comply and move on.