Based on a new proposal by the Belgian Minister of the Interior, EU governments could endorse the much-criticised child sexual abuse regulation (CSAR or chat control) as early as June after all. This is what internal information leaked to Pirate Party MEP and shadow rapporteur Patrick Breyer reveals
If this passes, could you not self-host an older version of an open source service that does not contain the backdoor (e.g. Matrix) for your closests contacts to circumvent this? Not saying that would be very practical for all communications, but at least for exchanging nudes with your partner? If so, at least there's that, but it would show how useless it is likely to be as anyone actually in the stated target audience could do the same.
Or is there something I'm missing that would prevent it?
Why necessarily an older version? Do you think a XZ-like well-hidden backdoor in some update is likely?
It is assuming this is implemented in a way that forces all existing messaging services to implement this or shut down. In that case, you would want to build it from source from a point in time before it was implemented (or shut down). If that is not the case, then this wouldn't be much of a problem to begin with, right?
I mean, running outdated software is bad practice. Do you think backdoors would have to be cleverly hidden every time, like what we almost got with XZ? If it is in plain sight, I highly doubt a person outside of the oppressed jurisdictions (or just someone anonymous) wouldn't make a malware-removing fork.