FWIW, I'm very sure third-party apps are already legal (and have always been legal): the issue was never that you aren't legally allowed to make them, it is merely that, once you do, the company making the product goes out of their way to break your client or ban your accounts; so, what you'd really need to do is to make it actively illegal for a company to prevent you from making a third-party app... even by any such technical means that they will absolutely defend as some form of "spam prevention" or "security mitigation" or "content restriction".
What I mean is get rid of any way that the legal system can be used to prevent third-party apps. TOSs, CFAA, DMCA, contracts, etc. should all be null-and-void when used to prevent interoperable third-party apps.
Companies with large market share should also be actively required to facilitate interoperability, e.g. with APIs available on FRAND terms.
I am very confident the DMCA already doesn't apply even if this were a copyright issue (which I don't think it is) due to interop exemptions, terms of service generally aren't legal issues in the first place, and the CFAA doesn't apply as there is case law that this is not "access". I maintain: there already is nothing stopping you from making a third party client except 1) how people keep making the stupid mistake of violating trademark law in their third party client (which is trivially avoided) and 2) the legal right of the service to refuse service to anyone... you need to make that actively illegal, and then defend that that makes sense against the onslaught of arguments that they need to be able to ban people due to spam prevention, security mitigation, and content restriction (and the EFF coming out of nowhere with some kind of argument that this is going to undermine the right of free speech of the company due to similar arguments as to Section 230, etc).
You would also need to mandate that they supply a reason for banning you that you can argue against. As it is now you could just get banned with no recourse or reason for using a 3rd party app on many platforms.