This is still a recognized command (for legacy purposes), but should be avoided. See Does it matter if I use
\bf, etc. and Will two-letter font style commands (
\it, …) ever be resurrected in LaTeX?.
TeXstudio does allow you to add commands that are not recognized to its list of known control sequences. Under Options -> Configure TeXstudio... check Show Advanced Options and add
\rm to the list of Additional Commands as part of your Custom Highlighting:
Your document syntax highlight should change from
Another approach (perhaps better for legacy documents) is to replace any two-letter font style commands with their updated counterparts:
This renewal will also be picked up by TeXstudio.