So, for example, it looks like this protocol would exclude people getting chemotherapy.
Happy to be proven wrong on this by someone with more information.
I don't know how many people with conditions take immunosuppression medication. I take it for arthritis and alopecia and have come across many people on similar meds for diabetes or other things. But that could be a form of confirmation bias
People experiencing long term symptoms? I can believe that, my tinnitus after covid-19 is still there, even if I don't notice it as much.
Actual viral infection? I really doubt that, thoug perhaps it could hide like the herpes virus.
At the moment it's all quite nebulous, with a lot of anecdote and not enough data. But there are some studies starting up that might start to untangle what's going on.
My inexpert understanding is that mRNA vaccines such as this one are unlikely to have an impact on many autoimmune cases. In many, perhaps most, others, it appears from my reading to be likely that the generated interferons are not adding anything to the likelihood of developing an autoimmune condition that wouldn't eventually already happen. (It might be the nudge over the cliff, but so might have the next common cold you catch.) But it should be tested to understand, regardless.
There are other reasons a person would be considered immunosuppressed, and plenty of non-chemotherapy drugs that have immunosuppressive effects. But yes, cancer is one of them.
It would make no sense to include immunosuppressed people in the initial trials for a vaccine, the purpose of which is to determine whether a vaccine can induce an immune response.
My selfish concern is then whether or not the vaccine will work for them.
We won't have hard data on that specific question for a very long time, but it's something your/their doctor would be better equipped to make an educated guess about, given the specifics of their situation ("immunosuppressed" can mean a range of things). In some cases, a vaccine might do nothing. In some, it might be actively dangerous. In some, it might have a weaker but still nonzero protective effect. But I wouldn't trust anything you read on HN that tries tell you anything more specific than that.