What is the current behaviour and why should it be changed?

The current code selectively disables clang-format and deliberately un-indents code which is only kept for compatibility reasons.
The idea is obviously that this should make the code stand out.

This has downsides:

• readability
• specific instances confuse clang-format despite the disabling (see clang-format issues settings.cpp #2587)

Describe possible approaches
It would be good to find an alternative approach here which keeps the regular indentation.
One possible idea from https://github.com/jamulussoftware/jamulus/pull/2587/files#r845895199 is using #defines.

global.h:

#define COMPAT_JAMULUS_BEFORE_3_6_1

somecode.cpp:

#ifdef COMPAT_JAMULUS_BEFORE_3_6_1
if (someConditionToCheckOnlyForOldJamulusVersions)
{
#endif

  someRegularCodeForAllVersions();

#ifdef COMPAT_JAMULUS_BEFORE_3_6_1
}
#endif

Another approach is a style using regular TODO comments (see below discussion for examples).

Has this feature been discussed and generally agreed?

No, needs agreement.