There’s a subset of the polyamorous community who aren’t able to be open about their romantic lives in one or many areas of the rest of their lives. Either they have family who they think won’t understand who they’ve chosen to be with and how; or they work in a job where the fact that polyamory isn’t a protected class is a real problem; or they think that admitting they’re polyamorous will create enormous problems somewhere in their lives because people don’t understand what that means on a broad scale yet.
I think that last concern is becoming the least of a concern as polyamory gains social acceptance - as pediatricians, teachers and school administrators, and generally People You Have to Deal With acknowledge that polyamory is no weirder than divorced and repartnered parents, who they have to deal with all the time. Much like divorced and repartnered parents 30-40 years ago, there’s a lot of backlash from other parents and the occasional rude person, but by and large things are getting better, not worse, and will continue to do so. (Unlike 30-40 years ago, we have the internet in full so every rude person gets to be as loud as they want and as brave about it as they want so they get to appear to be more numerous than they are.)
The other two are very real still, and probably will stay so for a while. As fast as changes in protected class happened for LGBT people, because of changes in the makeup of the SCOTUS and the fact that those are lifetime appointments and the fact that there will be legal challenges to legislative protections, I think that it will take longer than any of us would like to get those legal protections to keep everyone safe at work. Without those changes and protections, there will be no way for people who are closeted because they’re afraid of losing jobs to be open about their polyamory.
Family who don’t or won’t understand will always exist. I’ve got a post about what I think about that as a reason to not do polyamory and not be open about polyamory. But, honestly, I used this as a reason for not being open with everyone (and said “oh it’s not a big deal because my family lives a long way away and won’t meet anyone except my live in partner anyway”) for several years, so I understand where this thought process comes from and how it happens. But it makes it harder later, both to talk to family who feel like they’ve been lied to for an extended time period, and for the partners who aren’t the “public” partners. I’ve been that “not public” partner in the past as well, and being the person who doesn’t get acknowledgement really doesn’t feel great. If you’re in agreement or in a relationship with someone who also has to hide that they’re in polyamorous relationships, it can be no big deal to have some relationships you just don’t talk about. If only some of you want that… that’s a conflict that needs to be addressed.
As you’ve maybe noticed from months of posts, my preference and suggestion would be to be open if you possibly can. If you really can’t - if someone’s job is on the line, if a livelihood will be ruined, if the whole house of cards will collapse - then it’s up to the person who wants openness to decide if the price of admission of giving that up is too high to stay in that relationship. Nowadays, that's too high a price of admission for me, though for many years it wasn't. Everyone has their own dealbreakers. Think hard and then let your (prospective?) partner know. This is an important conversation to have early on in relationships.
Unrelated but... I can't turn off my brain, so: It's Election Day in the USA, where I am. If you're eligible to vote here and haven't, please go do so today. This is an important one, even more than usual.