Is it bad practice to create a non user directory in /home?

I installed ubuntu on a computer and I have the OS installed on a solid state drive, and the home directories mounted from a separate hard drive. I wanted to create a folder proj which lived on the second hard drive, but I hadn't partitioned it, so I just used mkdir /home/proj.

proj is meant to be a directory which contains various software packages used by different experimental groups. For example, /proj/experiment1 would contain software used by one group and proj/experiment2 would be different packages or versions used by another group. I wanted it on the second hard drive with the /home directories so that the OS can be reinstalled while keeping this directory intact.

I'm now worried about what will happen to this directory if someone in the future runs adduser proj. Will this delete the directory and replace it with their home directory.

Is it bad practice to create non-user directories in /home? If so, what should I have done?

Multi-user, shared modifiable directories are, by convention, usually somewhere in /var or /opt.

However, if you are really worried about someone making a user called "proj", you could have just created a proj user, set its account disabled so nobody can login as it, and put the files in its home directory. Then make the home directory world readable and executable (and writable, too, if you want that).