The ECHR is completely separate from the EU so no matter what way the actual Brexit goes, they will still be a member of the ECHR. Whether they might want to leave that as well is another matter.
All people hear are "European" and "Court" and assume its part of the EU and Brexit means the end of that so we can get back to torturing and executing like the Good old Days...
I would say the European Court of Justice is also outside the EU as it also regulates the EEA, but Brexit has meant leaving that too for about two years now.
I know it seems weird and cynical to say a government might not understand what it is and isn’t leaving, but my total lack of faith in the UK government at this point means I’ve chosen to leave the country rather than risk it.