Thank you @Anonymous User. That use case is centered primarily around the idea of best practice design to not have multiple fields for essentially the piece of information. The Notes field example is a classic case. We often add a Notes or Comments field to feature classes or tables to capture the non-standard, free-form info needed to help explain a particular issue/situation ("Did not inspect. Angry hedgehog has a burrow adjacent to facility." *Not a real note!)
Your Includes() function solution appeared to be a reasonable alternative, so I dove into the Smart Form to implement it. That is when I noticed that the Includes() function wasn't in the list in the Arcade Editor. Hmm. Diving into help there was a link to the version matrix, where I found that this function isn't available until the 10.9 release--and we are on Enterprise 10.8.1.
Before posting my question I came across a post that used the IndexOf() function. It appears to do something similar so I looked further into it. In the end, I was able to make the IndexOf() function work for my purpose.
For others' benefit, this is my syntax for the Notes field, which needs to appear for every activity in my domain list:
IndexOf(['Register Replacement','Radio (MXU) Replacement','Meter Install','Meter Replacement','Meter Removal','Other'],DomainName($feature,"WorkType")) >= 0
Note: You use the domain's description and not the code for the list.
My Smart Form is now working exactly how I need it to function! I believe the Includes() function would have worked well had I already been using 10.9.
Now my only outstanding issue is that I get an "Unable to Submit, Unable to complete operation." error when I attempt to submit my form within Field Maps. Hopefully I can track down the cause of that soon.