"undesirables" in this context refers to people whose ideology was seen as so incoherent and irrational that it could only be explained as a form of mental illness. there was this idea that fascists and the like were just fundamentally damaged people who required treatment, not punishment. but that treatment quickly became just another form of punishment.

this punitive approach to psychiatry was ... well, it's an incredibly problematic approach, to say the least, and not only because it comes with a stigmatization of mental illness and other deeply ableist ideas. there's a whole other can of worms here.

on one hand, i actually do think that a lot of nazis, incels and terfs are traumatized, abused, deeply hurt individuals who have built their ideology on the foundation of the untreated psychic damage inflicted on them, that their ideology does constitute a symptom of their trauma that has taken on a life of its own and that re-educating them is only possible when you adress their trauma first. and obviously, rehabilitation would be preferrable to mere punishment. their trauma doesn't excuse them in any way, but it does explain why they are the way they are. these people do need treatment.

but ofc such treatment is only possible when the patient complies and cooperates. you cannot treat mental illness against the will of the patient. that's not only deeply unethical, it is outright impossible that such a forced therapy approach could ever be successful. therapy requires that the patient heals themselves. the therapist's assistance can be absolutely necessary for that, there's stuff you just can't work through on your own, blind spots, things that are buried so deeply you need help in accessing them, but all of that invaluable assistance is just that, assistance in the patient healing themselves.

so yeah, this whole idea is something that we as socialists should take as a warning, as a historic mistake and as a lesson to be learned. we cannot heal our enemies against their will, even when they actually would need healing.

the only way to go about this is to offer free and easy access to good, compassionate mental health care to everyone. i think when we, as outside actors, instead tie the ideology of some of these patients to their trauma instead, we actually make it harder for them to seek help, because then seeking treatment becomes something that attacks how they view themselves. this can only cause them to double down, to resist the help that they need and to preserve their reactionary ideas.

tl;dr: punitive psychiatry was a grave error that should not be repeated in future socialist projects