Jep, some software titles (still) require node 8, so within dietpi-software we manually downgrade via
npm i -g --unsafe-perm n && n 8.
Not sure if users will even recognize if their node version changes, but jep a warning would be an idea, at least if Node was already installed before.
However, AFAIK when installing Node relying software, Node + modules are installed locally within the software install dir? But not sure and I think depends on the exact install method

.
However it should not matter, since all software should as well run with Node 8. Would be a shot in the knee to drop LTS version support

. Also AFAIK the old (respectively new v10) should be still available. The n versioning AFAIK places different versions beside each other. But not sure if this means that e.g. node-red will remain using v10 or always uses the one that was linked to the $PATH dirs (so v8), or even a locally within software install dir placed one. Should be also different depending on how the software and it's dependency node modules were actually installed.
Simply do a
dietpi-backup and try it out. If anything fails, revert.
Hmm
npm i -g --unsafe-perm n fails in your case?? This is strange, since it works well during our installs. Also we do not install npm/Node itself via n at first. Assure that you run it as root or with sudo.
Or is in your case Node not yet installed? Of course npm is only available if Node is installed first

. In case you can install it via dietpi-software > Additional software (I believe) and then you can use n 8 to downgrade. Or let me check how to directly install Node v8, which should be cleaner of course.