Actual transparency, in practice. Unless it's an org focused around an open-source component, it doesn't have to be IMO GitLab's[1] level of radical _public_ transparency, but it should be at least as radical internally.
Even partial transparency was useful when everyone was worked in person, because gaps were easier to mitigate via "you didn't hear this from me, but" statements from immediate managers, or from peers in other teams, or from just the uncomfortable situation of showing up in person and asking about it directly. But working remote-first or -only, then actual, practiced transparency is an absolute requirement for me. (Which sucks, because that's very difficult to suss out from a typical tech interview process.)
I've worked with two remote companies during the pandemic that've struggled with this. It's always listed as a "company value" but sometimes comes with a fundamental misunderstanding where "transparency" is only openness - being willing and wanting to answer any questions when asked, which is also good. But actual transparency is both openness AND having the discipline and process to document everything important - good and bad, erring on the side of oversharing - and share it with everyone _without being asked_.
This requires practice, discipline, common venues and processes, and a blameless culture. It empowers every other thing I also value in a company - transparency doesn't just encourage me to talk about what I need, it requires it, and makes it communal by default. So I might feel uncomfortable being transparent about telling people what I truly, honestly, frankly want and need out of work in terms of work/life balance, pay, advancement, training, joy, space, management, teamwork, and process, but so does _everyone_, and in the end there's far less ambiguity about where I and everyone else and the company all stand on them.
I can stop worrying about the things I don't know about the company because it's all just... there. Less ambiguity means less fear, less friction, less bikeshedding. We know what we have, what we need, and where we're going. Actual internal transparency up and down the organizational ladder flattens it for the important part - the impact that any one person can have on the company - even if it isn't structurally flat for making decisions.
In a mediocre org, a win gets celebrated in a team. In a good org, a win gets celebrated across a department, maybe even by everyone in a special event. In a transparent org, every win gets celebrated by everyone. The same works in reverse - when a team member struggles, at work or in life, everyone can help if everyone knows about it. I learn to shed fears of failure because everyone's failures are on display, and everyone's getting help to solve them, to do better going forward, and everyone shares the responsibility and accountability of doing so.
When false transparency comes without accountability, it never moves toward transparency, which lets things that rot the culture slowly and seemingly mysteriously from within fester, in secrets and silos and forgotten corners of the org. It's painful to have worked at an internally transparent org and then move into an org where different teams are at each others' throats, not because of personal conflicts but because one team has information that the other one needs to work, and they can't get it because of arbitrary obstacles that practiced transparency would eliminate.
It isn't perfect - it's uncomfortable, it's overwhelming, it's incredibly hard to effectively scale to dozens of new people per week as successful startups tend to do when they land some funding. It certainly isn't for everyone, but I also often find that the people who feel the _worst_ about transparent work cultures and practices are people I work the _least well_ with.
[1] https://about.gitlab.com/handbook/values/#transparency